In the first tier of raiding in Catalcysm, tier 11, I almost never used Heal. It was just too slow and weak. It was like a caster DPS blasting away with her wand. It will have some effect, but not enough to matter.
However, I'm noticing something interesting at the start of this new raiding tier: Heal isn't half bad. It's still much, much weaker than Greater Heal, Flash Heal and Penance...but it's actually useful now for a few reasons.
Heal Has Been Powered Up By Gear
One is that with gearing getting better, the speed of the heal is no longer crippling and the size of the heal now has the potential to move raid member's health bars (even if not by a huge amount).
"Sure," I hear you saying. "Heal is better with better gear. Big deal. Every heal is better with better gear, and everyone's health is increasing too. Holy Nova is better now, too. Time to use Holy Nova?"
No, of course not. Don't be silly. There's never a time to use Holy Nova unless it's to run around the Molten Front dailies, spamming it so that you get kill credit on everything.
However, throughput gains from intellect, haste, crit and mastery seem to be scaling with gear more than health pools (and, therefore, boss damage since boss damage output has to be firmly correlated with health pools*).
*Theoretically, boss damage output could be untethered from health pool size...but the only way for that to be viable would be if healing spell output were utterly massive, which would still mean Heal's output would outstrip health pool scaling and move life bars nicely.
Therefore, Heal is actually more powerful relative to this tier than it was relative to the last one. Sure, Flash Heal, Greater Heal and Penance will always do much more, but Heal is cheaper. Which means...
Healer Auto-Attacking (Or Wanding) Has Value
Any time damage is pouring in in a big way, you'll have other priorities, like the aforementioned bigger spells as well as Binding Heal and Prayer of Healing. But at the start of this tier, mana efficiency is going to matter again. Perhaps not quite so much as at the start of the last tier (since base mana didn't go up, so spell costs remained the same while mana pools are increasing) but with incoming damage higher, your larger mana pools will be challenged again.
And if you're a healing priest, Blizzard has a special gift for you: exactly zero of the cloth drops in the Firelands have spirit. That's not to say spirit doesn't exist for us; you can get spirit on valor point gear and there is spirit on some of the trinkets and jewelry (that every other healer will be after too)...but at least for a while, you may be packing fewer spirit pieces than you'd prefer.
Therefore, you want a tool that can have an effect without affecting your mana much. Enter Heal. The spell is at the very least mana neutral and may even be mana positive depending on your spirit levels and any mana regeneration tools occurring at the time (for example, Replenishment). By "mana neutral," I mean that you regain Heal's mana cost during the cast time and by "mana positive," I mean that you regain more than Heal's mana cost during the cast time.
This means you can still make a contribution essentially without expending mana or while (more slowly) regaining mana. This is useful during periods of low damage, to marshal your resources for the more intensive periods, and for when you run yourself right out of mana. Sure, being limited to Heal makes you a minor contributor at that point, but at least you're still in the fight rather than an interested onlooker.
Let's also remember that Blizzard's little surprise for healing priests (no cloth spirit drops) has a benefit: It means more pure throughput stats, since that stat budget has to be filled up one way or another. Now, until spell costs become completely trivialized, I'm a big believer that spirit is the best secondary stat because it can either give you mana regeneration when you need it, or can be converted into throughput when you don't. That said, spirit is definitely not better for pure throughput than mastery, haste or critical strike. Having more of those stats will make your Heal spell (in addition to all your others, of course) even stronger, allowing you in a strange sense to convert throughput stats into mana efficiency: You can cast more Heal and therefore conserve mana.
Critical Strike: Now With More Critical Goodness
As I'm sure you're aware, the latest patch brought with it a boost to all healers: critical heals now provide double the original output instead of one-and-a-half-times the original output. That means that when you drop in a critical strike Heal, you'll see a surprisingly large number. I've had Heal hit for 20k+ in heal-y deliciousness. That's pretty not bad for essentially no impact on my mana bar.
You can't count on a critical strike, but you can cast Heal on a target, hoping for a critical strike or two, until the target (or another target) is beginning to look a little green around the gills, in which case you can switch to bigger heals. A run of two or three critical strikes can result in fairly legitimate healing, leaving you awash in mana (which is blue and cooling to the mind and skin).
Tier 11 Two-Piece Bonus: How Ya Like Me Now?
It's still not exciting. That said, it's actually relevant now. I completely ignored the set bonus during the previous tier because I never cast Heal during raids, unless it was while fighting trash and everything was under control with little healing to be done. So the set bonus had no benefit at all for me.
Now, however, until you replace at least three pieces of your tier gear, this bonus will help. As we just noted, critical strike Heals can make a real difference and a 5% higher chance of getting them is a non-trivial effect. It seems like a small difference, our effectiveness is measured in a percentage here and a percentage there. It's a small chance for chaining a couple or three critical strike Heals together and those add up.
Bringing It All Home
Don't forget that Heal (like Flash Heal and Greater Heal) will reduce the duration of Weakened Soul, allowing you to get a Power Word: Shield back up on that target sooner. Similarly, Heal (like Flash Heal, Greater Heal and Binding Heal) has a chance to proc a free and instant Flash Heal if you have the Surge of Light talent. In both cases, Heal is a cheap way to take advantage of those talents, allowing you to more effectively use more powerful abilities.
The main thing to take away is not that Heal is suddenly an incredibly powerful tool. It isn't and is not meant to be. Rather, it's now a viable tool in the toolkit when raiding, rather than merely something to use when things are stable and boring in dungeons. You shouldn't be leaning on it, it shouldn't be a large part of your healing or usage...but it should be used now. It can provide value, essentially for free from a mana perspective. It's drawback is opportunity cost...it's a piece of time that you're not using to cast a more powerful spell. Most of the time, you can't afford that opportunity cost...but there are times when you should.
Casting Heal is like kissing your sibling. It's not generally recommended and feels a little shady. But in the right circumstances, especially with alcohol involved, it doesn't look half bad and everyone goes to sleep happy.
I can't see any flaws to that metaphor at all.