"Shield spam" has become a term of ill-repute in the discipline priest community. It's spoken of either in whispers about the "bad old days" or else in the sharp, mocking tones of what not to do. Only the most degenerate discipline priests would encourage shield spam. It speaks to loose healing morals and, yes, even a bit of healing perversion. Shields are to be enjoyed in small quantities, not to gluttonous excess.
That is why I'm here to tell you to reject those stereotypes. Even good-hearted people can blast out bubbles. It's not the evil that you've all heard it is. It can--nay, should--be done. Don't listen to your neighborhood priest telling you that it's sinful.
That said, we should probably define what we mean by "shield spam" because multiple behaviors have been lumped into that one term and they are not all the same.
Let's start by examining where the term really sprang into prominence and trace our story from there. This, it turns out, may run a bit long but I promise there's a point at the end. And a pizza party. Pertinent point and pizza. And cold beer.
Come with me, then, back to those heady days of Wrath...
Spammy Shield Spam
Welcome to Dalaran! Do you have any emblems of frost to turn in? And how about those vrykul, eh?
I hope I've adequately set the mood. Back in this era of World of Warcraft, discipline priests were slowly but surely rising in the world of healing. In prior eras, discipline was really more of the PvP spec and holy was the healing spec for priests. However, the tree was revamped in Wrath and PvE discipline priests were becoming more and more populous.
Oh, sure, there was still residual persecution that they faced. They were regarded with suspicion, not being holy. Absorbs were a bit of a new mechanic to much of the playerbase and no one really paid attention to absorb values, which made discipline priests appear to be rather ineffectual, what with Power Word: Shield and Divine Aegis being completely ignored by almost all healing meters.
However, as raiders were graduating from Trial of the Crusader to the final raiding tier of the expansion, Icecrown Citadel, people were understanding the power of the absorb. High-level players had understood it long before, but it gained popular acceptance. Now, discipline priests were not just being accepted but often actively recruited as they carved out their own niche. Discipline priests weren't healers, they were shielders! You could almost think of them as a huge damage debuff on each and every boss.
This was a double-edged sword. It made discipline priests a rather necessary part of any raiding team capable of picking and choosing their raid compositions, as much so as getting a shaman in for Bloodlust or having a druid who could Rebirth. Having such a valuable niche in raid teams is obviously a nice thing...damned are the poor souls who play class-specs marginalized by current raid design.
On the other hand, it was a terribly boring playstyle, especially in 25 man raids. In 25 man raids, very literally a discipline priest's job during an encounter was to do nothing but blanket the raid in shields. And then do it again. And again. Until the boss was dead. Other healers were there to do the actual healing, after all. In 10 man raids, it was a little better, purely because there weren't enough people to shield before the Weakened Soul debuff prevented you from casting more shields. At that point, it was reasonable to cast an actual healing spell.
So discipline priests were in a bit of a bind...the niche that gave them ready access to raid spots also trapped them into a rather unenjoyable style of playing. (This wasn't purely confined to discipline priests, as I know there are restoration druids out there who have nightmare stories of being Rejuvenation bots. But I think it was most pronounced for discipline priests.)
Our Story Runs Into A Twist
Blizzard agreed that it was a trap. Their design philosophy had been slowly attempting to weed out repetitive, or One-Button, play. Discipline priests sometimes using nothing but one spell in raids didn't escape their notice. So they changed it. So mighty was their change to the discipline priest spec that they named an entire expansion for it: The Cataclysm.
Perhaps you heard about it? The change resulted in many fires around the world.
What they did was make Power Word: Shield really weak. It was brilliant. Why would you spam a spell that wasn't very strong? You wouldn't. That Blizzard, always thinking.
Well, as it happened, trying to reduce a spec's dependence upon their best and most iconic spell by making it terrible didn't do wonders for balance, that nasty issue that boils down to "Do you get to raid or not?" Discipline priests were pariahs again. Go holy or go home. The chants were hurtful.
So Blizzard did what any wise design team would do. Turn the dial the other direction as far as it would go. Suddenly, Power Word: Shield was really strong. Discipline priests were great again...in fact, they were recording by far the best parses overnight. No other class could compete.
"Hmm," said Blizzard. "That was unexpected." Also, there was this issue of discipline priests spamming their newly god-mode spell on everyone again. Also unexpected.
So now Blizzard needed a solution that did not involve turning that dial at all. Turning the dial on Power Word: Shield, you see, revealed a problem inherent in WoW balance. A spell you can use whenever you want will either be strong enough that you will use it first, last and always or else so weak that you will never use it. For if it is the strongest spell right now, it will probably be your strongest spell a moment from now. And if it is not your strongest spell right now, it probably won't be your strongest spell a moment from now.
The answer resided in the other dial that Blizzard had ignored in Wrath (which caused that previous logic about "strongest spell now->strongest spell always" to be binding in nearly every case) but used with abandon in Cataclysm...the mana cost dial. True to the Cataclysm healing model, which held that you don't simply min/max based on power of spell but min/max such that you don't go out of mana in the first minute, they increased the mana cost of Power Word: Shield greatly.
That changed the game significantly for discipline priests. The shield was still really strong, and it gave mana back every 12 seconds, so it was still useable...but "shield spam was dead for good" (I place that in quotation marks because it was probably the most common quote of the past year, it even featured on the cover of a Time magazine).
This was driven home with a peculiar sternness on forums and guides for discipline priests. Perhaps the people saying it felt it was necessary to say it with passion to break the dirty habits people had built up, but a great deal of scorn and derision was heaped upon the concept of shield spam (in my experience, at least). Not that it really required stern insistence...any discipline priest that tried to spam shields during a raid encounter, Wrath-style, would find themselves as merely a decorative prop pretty quickly, completely out of mana (OOM) and therefore unable to help much. This was a lesson as subtle as a jackhammer to the head.
And lo...it is still impossible. Unlike the latter stages of Wrath, where no spell could really run you out of mana, casting shields non-stop on a raid group, even in 10 man, will solidly OOM you.
So What's All This Talk Of Shield Spam, Then?
The problem is, "shield spam," which was originally applied to casting shields on each raid member robotically, over and over, is now being applied to any behavior involving using lots of shields.
The fact of the matter is that absorption is still the healing space of discipline priests. That is where you can add the most value while still allowing all other healers to do what they do best. While holy paladins have entered the space to some extent (to say nothing of the abominations known as blood death knights), it's still an important niche that discipline priests largely dominate. Damage prevented or mitigated is more valuable than damage taken and then healed up (the easiest way to see this is to compare the value of absorbing a 30k hit to a player with 24k health remaining, versus attempting to heal it up after).
While Divine Aegis does yeoman's work in terms of absorption, there's no faster way to get on-demand absorption out on the raid than what you might call "spamming" shields.
The key here, of course, is that you must choose when it makes sense to spam those shields. Your mana won't support doing it all the time. And the mechanics of bosses (along with a duration lowering change that I neglected to mention earlier) are such that ill-timed shield spam will come at great mana cost for no benefit as the shields mostly drop off due to expired duration without absorbing any damage at all.
So you absolutely have to pick your spots intelligently. However, if you do, there's a world of benefit to be mined from strategic shield spamming. Getting ten shields out ahead of a big raid-wide damage spike can effectively increase your raid team's "clutch heals per second (HPS)" by which I mean that if you're using GCDs during a low-stress period to put up shields that will be eaten by a high-stress period, it frees you up to cast other spells during that high-stress period. You're borrowing time from a low-stress period to lessen the difficulty of a high-stress period, in a way other healers largely can't. Absorption shields are about the only "preventative healing" (though HoTs can be used in a somewhat similar way, though less efficiently for the same purpose).
You won't be able to shield to the exclusion of all else, even if you pick your spots. There are times, especially in 10 man raiding environments, where helping your fellow healers move life bars will be necessary. However, the more you bias your casting patterns in favor of Power Word: Shield, the more you can help your raid team even out the spikiness of the overall incoming damage of the encounter. It also has the benefit of clarifying your stat choices for gemming, enchanting and reforging. If you know you will be using as much of your mana as possible (over the entire encounter) on Power Word: Shield, then you know mastery is your best bet.
This is Smart Shield Spam(tm). This is good. This is a perfectly responsible way to play discipline priest. There are other perfectly responsible ways to play a discipline priest, but don't let others lazily say "Shield spam is bad, mmmkay?" Make sure they define what they mean by shield spam...if they're talking about Wrath-style bubble botting, nod at the obviousness of their observation. Yes, you can't do something that you don't have the mana for. However, if they're advocating shielding as little as possible (some even claim you should largely only be shielding the tanks or only shielding to proc Rapture) then reject that supposed truism. Intelligently spamming shields at the right times can be an extremely powerful way to play (I would argue the most powerful way to play)...you should experiment and see how far you can stretch Power Word: Shield, especially as you push into progression fights that stretch you to your very limits.