The important part is the stability - if your usernames can change then they aren't stable so you don't select it. I think it is a good reminder that most thin

The important part is the stability - if your usernames can change then they aren't stable so you don't select it. I think it is a good reminder that most things you think of as being unchanging that are also directly related to a person.. aren't unchanging. Or at least any conceivable attribute probably has some compelling reason why some one will need to change it.