Sorry Ghostcrawler, but this is bull.
That links to a post where the developer flat-out announces that the buff "replenishment" (which gives bonus mana regen to a portion of the raid) is required for raiding. So after all that talk of "bring the player not the class", it comes down to this. It's obviously wildly hypocritical, but they don't want to bother to fix it.
Why is it hypocritical? Because replenishment is only available from a tiny subset of a small group of classes. There are 30 talent trees in the game. Only 3 of them offer replenishment. 2 of those are rare-in-raids dps specs of classes that can also heal and/or tank (preist and paladin). Shadow priests are known to be weak right now, and I can't remember the last time I saw one. Ret Paladins are PvP monsters, but you don't see them as much in raids because they are too busy killing people during a stunlock (something that isn't possible for most rogues, a class built around stuns who also don't have plate or an invincibility shield, but I digress) in arenas and BGs. Until the most recent patch, Survival hunters were also rare because Beast Mastery was universally acknowledged to be the dominant spec. Recently, Survival was buffed to come out ahead, which helps a bit.
In fact, my guild was without replenishment until that patch got our hunter to respec from BM. Right now, we are small, focused 10-man raiding guild. We have no paladin, one hunter, and one priest (who is one of our 2 most depended-upon healers, loves healing, and hates shadow). If not for the buff to Survival, we would have been put in a lose/lose situation. We don't want to recruit someone we don't like (in fact, we have no desire to recruit a stranger into our close group of friends at all!), but we also don't want to be gimped on every raid encounter because the devs had to make replenishment so rare.
But basically, GC just said "Sorry, 10 man raids, but you have to bring at least one of these to your raid. What if you have no hunters and are relying on your palading to tank and preist to heal? Well tough luck, you're screwed, and frankly we don't care. In fact, we intend for you to be screwed. Remember that whole thing we said about bringing players and not classes? Well we didn't really mean it. Now get to work recruiting a terrible, annoying player who happens to be a Ret paladin, and stop whining to me on these forums about how this is hypocritical."
Sure, there are many other buffs limited to a small subset of specs. But hey, guess what? None of those are required!
GC, if you and your development team are really serious about allowing replenishment to be "mandatory", you have to extend it to a wider variety of specs. You don't have to give it to an entire class, and you don't have to give every spec of the current classes the buff if you don't want to. You just need to make it so the buff isn't languishing on 2 obscure dps variants of healing/tanking classes and in one tree of three for hunters. Having just 3 our of 30 specs provide the buff is too punishing to ten-man groups. There is absolutely no reason not to either add the buff to other classes in one tree each, or to give the ability to more trees within the current classes that have it. Make them deep, expensive talents if you want.
If you don't want to spread them out to much, here's what I'd suggest as a compromise, in 3 simultaneous steps:
1) Give replenishment to all hunter specs. Make it a deep tree talent in all of them.
2) Leave replenishment on Shadow priests and Ret paladins
3) Give replenishment to some more widely-used dps specs: warlocks (either Affliction or Demonology) and mages (Arcane)
Suddenly, instead of 3 our of 30 specs, you've got a much more palatable 7 of 30. And you'll notice I went out of my way to make them "make sense" (warlocks sucking mana, arcane mages giving out mana, all the classes I suggested use mana themselves).
Update: WoWInsider agrees. In fact, a randomly-selected 10-man raid currently has a full 40% chance of not having a single replenisher.