Most of the web sites I use regularly use an even better solution: your email address is used as your user name. How could it be better? A valid email address is surely unique, and the risk of it being forgotten by the user is sufficiently low.
Sometimes an e-mail address isn't enough because if the site has any type of social aspects to it you wouldn't want your e-mail address being used as your identifier.
I agree though for sites where you're not mingling with other users or have any notion of a public profile then a username at this point is pretty silly.