Why not just make everyone a DPS?

[Yorumi]

Same can be applied to almost any other PvE (and even PvP...if you're good enough :P) scenario. You're essentially repeating the great PvE vs. PvP debate.

Not really. PvP is the opposite end of the spectrum but there's a lot inbetween. For example classes can be done with more a priority system than a fixed rotation. A priority system isn't mind blowing by any means but makes things feel just a little less scripted. Bosses can just add little bits of rng to the fight, not to the degree you're winning or losing fights on rng, but enough that it makes it feel like maybe the boss is thinking a little bit about what it's doing.