1) Buffs are breaking the game. There are many examples you could insert here, but here are a few:
-- Ret paladins can’t get into the melee group because of all the buffers in there.
-- Stacking shamans for Bloodlust rotations.
-- Stacking Shadow Priests so mana conservation would be irrelevant.
-- Mages getting shut out of raids, in part because it was better to bring yet another lock to benefit from Curse of Shadows.
2) We decided to consolidate buffs such that similar ones would not stack, but we extended the effects to raid wide.
3) Thus, in a 25-player raid, you need far fewer than 25 players to get the buffs you need, allowing you to fill those remaining slots with who you want.
4) Without the benefits of those buffs, those remaining slots would most likely just go to pure dps classes. Shamans, for example, might go from 5 per raid to 1 (or 1 death knight instead).
5) So we buffed hybrid dps a lot.
6) But we knew at the other extreme that pure dps classes risked getting shut out of raids if hybrids could do their damage.
7) So we made sure the pure classes could still do superior dps given similar skill and gear. We wanted you to risk having lower dps if you brought all hybrids.