I don't remember who it was that said it, but I think it strucks a good point. "Trinity wasn't created by the developers, it was created by the players", to optimize a party set up. Why have everyone be a half-assed at everything, when you can stack all the defensive stuff on one guy, all the healing on another and all the attack power on the rest? It's more efficient and it's the natural evolution, even in nature and society, when units of the society specialize in a more narrow field to enhance the group as a whole.