This could cause problems with popular/unpopular users who delete their accounts. What if someone else used their username and then everyone already had a high/bad opinion about them. Also, what about groups, if someone creates an account, gets banned from a group. When someone else tries to join the group under that username they wouldn't be able to. I think to avoid this wew could have something in the users profile that shows if the account has been deleted, unfortunately not look at profiles. Another option would be to have the username not be useable for a couple of months.