Well, my fundamental premise is that you're not going to have a god-wizard or a CoDzilla because unleashing a big bang in round three is going to be roughly as effective as releasing three standard attacks over three rounds.
Mathematically, making 3 attacks that do X damage and hit half the time, vs unleashing one 3-round attack thad does 3X damage and hist half the time, would, indeed, be identical.
However, over the course of three rounds, a character might be dropped, dazed, stunned, or lose LoS to any valid target of the warmed-up power. While the character attacking each round would have gotten a hit or two in, the warm-up-power character gets nothing. So the power would have to do /more/ than 3X to be balanced. How much more? Hard to say, depends on all sorts of things, including DM & player styles. /Making it quite difficult to balance./
It's a matter of personal preference. I'd rather have classes be more flexible that have a plethora of new classes.
I guess the extreme would be one class so flexible that it can handle any character concept. I think Hero System has that class.

Seriously, though, I'm on the fence. I like the balanced foundation the AEDU structure with front-loaded class features provides. If there's a class that fits your concept, you're golden, and the game is nicely balanced. But, to handle more concepts you need more builds or more classes. More builds gives you more choices and potential syenergies within a class which can impact balance, or at least cause power inflation. More classes increases complexity, and causes class-related items, feats, and the like to proliferate, which, again, can cause some balance problems. Plus, if early classes have enjoyed some power inflation, new classes need to be beefed up a bit relative to what earlier classes were like at first release...
Not to be flippant, but ... because balance is no longer an issue?
I really think this attitude has sunk in. 4e is a victim of it's own success. By using quite strict design principles, it managed to be the best balanced version of D&D ever. Which is to say, it's the /only/ version of D&D to ever be genuinely balanced. That accomplished, balance has ceased to be a concern in the minds of many, so they're happy to throw it away while rooting around for something else.