I get the gameplay advantage of this, but I’m not sure how precisely this ties to anything that’s happening in the game world. Do you forget the power? Do you decide not to use it anymore? What happens if you come to need it in a particular situation? I can understand that you can get rusty at things from not practising them but you wouldn’t forget a thing altogether. Any input on these issues?

It has pretty much nothing to do with the fiction, from what I see. Deciding to not use the powers is probably the best choice, explanation-wise.

The point of flawed characters is that they are easy to poke (by GM or other players), which makes it easy to produce useful material in play. I’d say that all 4e characters are flawed in the good way, simply because there are the four character roles. It is difficult to fill more than two of them, even partially, making all characters dependant on each other in a way that is meaningful in D&D. (Some character being reluctant to take responsibility might not be relevant in D&D, for contrast.)