I've been a heavy user of power cards, but as my character(s) go up in level and the number of cards increases, I'm thinking of going with the power-cards-printed-on-a-sheet-in-a-wet-erasable-sleeve solution, just to keep the game table under control.
That said, though, that's really just an organizational difference. The broader point is that character management, for most people, has moved away from a conventional character sheet and into a format where powers are described in discrete blocks, and some sort of marking mechanism (turning over cards, or marking a sleeve with a wet-erase marker) is used to manage the resource.
Put another way: Nobody here is saying they just write a list of their powers on their character sheet, the way we might have listed feats or skills in an earlier edition. If we were to do that, gameplay would be much more complicated.
The complexity 4E introduced through the power-management system has been countered by a new paradigm for managing character capabilities.