Usually you end up with a jack of all trades that can't do anything especially well.
Personally I think we're forced to take too many powers as it is.
I understand what you're saying but when you have 18 main powers and then at least 6 pool powers, you get too many choices. It is probably easier for well-seasoned veterans to know what to choose, once they've mastered a specific AT, but for a newer player they would have no clue and we have seen how taking the wrong powers and/or mismanaging them through ignorance turns them off the game which is a bad thing.
If perhaps we had say 12 main powers and we could specialise in 6, then add pool powers we might find greater diversity but with 18 powers, some of which are specifically designed to be of greater power, it's hard to say we increase diversity.
I understand where you're coming from but in practice I don't think it actually is the case.