Fell Under your ideal, a group that takes a wiz and an enchanter will farm xp, money, or anything the drops per mob, at some fraction of groups that take non wiz and non enchanter. So the wiz and enchanter would only get invites for the content that requires them. That sounds like a much worse game to me.
Fair sustained dmg is required for all classes because the game is about farming, which is always a function of your dmg over the duration of the group. Not all locations in game will require Burst or CC utility for groups, but no class should feel like they cant get a group somewhere just because their utility role was already taken, or not needed. Classes be minimally effective to a group, even without their main utility role, by doing sustained damage. And that's great, it makes the game more social. I prefer being able to invite two enchanters or two wizards if two great players happen to be online at the same time. I wouldn't be as likely to do that if enchanters were only providing CC, or wizards only doing Burst with low sustain.
Then there's the problem of burst dmg vs burst aggro, like skoop mentioned.
Then there's the problem of potential for class stacking single encounters being OP when burst is high for one class, in a game where mana dumping exists as well, which will be a huge problem for designing tough encounters down the road.
