The problem is that if you play a pure dps class and are looking for a guild or are newly 80 and want to justify a raid spot it's hard now. Your argument used to be "Look, I'm good and can outdps a hybrid by a good amount" so you were recruited to increase the dps of a group by s significant margin. This isn't true in Wrath. Say you're a rogue - cat dps will come close to you and if the person playing the cat is open to it they can grab offspec gear for tanking or healing and dual spec... giving them flexibility in addition to dps that's basically the same as yours (assuming similar gear/skill from both players).

However, the solution isn't to hybridize pures, but to remember that pures bring other things to the tables and to design encounters to use those. Wrath has killed CC... but it's useful again on some Ulduar trash to sheep... that's something the mage can bring. Anestheic poison and Fan of Knives in Sarth is another.

In short, I think this is more a failure of encounter design than class design. Bring the player is good overall for the game, but the encounters need to get past the "pull, AoE, heal, pull again" mode and use the secondary skills that classes bring. CC, but also things like new poisons for rogues that are imaginative counters for the abilities of mobs, etc.