The only real reason I can think of would not happen very often. That reason would be: It frees the administrator from having to remember a username history in the event of trying to recover the users data in the event certain kinds of disaster recovery is needed for the users profile.

Like I said though, this would not happen very often. I get anal like this in my administration(when I have the time or when I dont think the change can be logically remembered by my feeble mind ).