Healers have long split stats into efficiency and throughput categories. Throughput stats were the ones that increased your healing done per second...the volume of healing you could push through any given time. Among secondary stats, these stats are/were spellpower, critical strike, haste and mastery. The efficiency stats were the ones that increased your healing done per mana spent...the healing longevity that your mana pool could sustain. These stats are/were mp5 (mana per 5 seconds) and spirit.
Mp5 is largely gone as an actual stat (though it exists as a concept, as things like a shaman's mana spring totem, Tyrande's Favourite Doll, the Corrupted Egg Shell and others essentially work as mp5 return), so we'll leave it aside. That leaves spirit as the single efficiency stat. And it's been a pretty important one for healers in the first few months of the expansion, as healing spells were far more expensive relative to mana pools than they were at the end of the Wrath of the Lich King expansion.
However, the logic for healers gearing through the previous expansion was expected to continue into this one. In fact, it seems too common sense to ever not be true: stack spirit until you feel that mana pool longevity is no longer an issue for you, then focus on throughput stats. After all, if you can last through a fight without being reduced to bandaging people, surely it's time to work on making your heals moar bigger?
I'm here to tell you that, while that may hold true later in the expansion, that almost surely won't be true in this tier of content. Even if you're healing heroics sitting at 90%+ mana and never running a risk of going out of mana in raids, it's unlikely you've outgeared the need for spirit.
The reason for this is simple.
For this tier of content, spirit is a throughput stat.
Spirit Is A Throughput Stat
Ask yourself this: In a raid, can you use your fast, inefficient heal exclusively? This spell is either Flash Heal, Flash of Light, Regrowth or Healing Surge depending on what class you are. Can you spam that spell as a solution to any damage that visits the raid?
The answer is probably of course not. (If your answer was "Sure I can," I'm happy to know Paragon's healers read this blog.)
In the final half year of Wrath of the Lich King, my discipline priest could use Power Word: Shield, Penance and Flash Heal (or it's hotter twin, Binding Heal) pretty much exclusively. I didn't need to resort to any other healing spells to protect my mana pool. That's a good reason to ignore spirit (or mp5 which existed as a stat then) and go for pure throughput stats.
However, right now, virtually all healers are constrained by their mana pool. The first raiding spirit benchmark, which is being met by many healers including yours truly, is being able to get through entire raid encounters without going out of mana, with efficient healing and intelligent cooldown management. This is being confused with a much higher benchmark, the point at which you should be switching from spirit to throughput stats (haste, crit and mastery--we'll ignore spellpower at the moment and get back to why a bit further down).
As long as you can't use your most mana-intensive and higher healing-per-second, your mana pool is constraining your throughput. That means that spirit counts as both an efficiency and a throughput stat. More spirit means better mana regeneration which means more of your better throughput spells can be cast. We are still in the era of spirit being the ultimate secondary stat...better mana regeneration will likely never be a throughput cap during this tier of content, as it was in the latter stages of Wrath. Better mana regeneration means getting to make a higher and higher percentage of your casts of the higher throughput variety. Fewer casts of Heal, Healing Wave, Holy Light or Nourish and more casts of your stronger spells.
So What's The Deal With Spellpower?
Spellpower is almost never a stat on its own (though weapons are the major exception) and it's not a stat we can explicitly stack as a secondary stat. It comes almost entirely from our primary stat of intellect.
Intellect is still our most important stat. This is because it is also both a throughput and efficiency stat. It provides spellpower (throughput), a tiny amount of critical strike (throughput) and increases our mana pool (efficiency, since there are key mana regeneration mechanics that work as a percentage of one's total mana pool). It's a more powerful combination stat than spirit and should remain so throughout the expansion and throughout the existence of World of Warcraft until Blizzard changes how stats work.
However, it hardly matters. Most of the time, you can't select spirit over intellect. Intellect, being primary, is generally budgeted separately from spirit, a secondary stat. Being primary, you cannot reforge out of or into intellect.
Gemming/enchanting are the only places where you could end up making a choice between them. Pick intellect. But pick spirit over everything else. It's a throughput stat right now. And an efficiency stat.
Wait until gearing has made the concept of mana the mere rumour that it was at the end of Wrath before you start ditching spirit.
(Pro tip: Remember to build a [Basic Campfire] in the middle of your raid. +4 spirit for free is basically #winning.)