I think it's wrong to make generalizations. (Yes, I know I'm stating the obvious , but I want to point something out.)

In some games, powerplaying doesn't hinder your roleplaying at all. Your intent may be "to build a powerful character", sure, but if (IF, I say, it doesn't always happen) you can combine it with good roleplaying which doesn't annoy your fellow-players and DM, who cares? Because that's where it all boils down to. Not to annoy anyone. Intentions are irrelevant.

But it some games, no matter how ingenious a player you are, you simply can't combine both. In some cases, sacrificing a bit of your mechanics' awesomeness is a prerequisite for a good story... And a good story, collectively created, is why some of us play in the first place.