Also, it prevents someone else from signing up with the same username! I saw this once on a forum where a particularly annoying person got banned and the admin deleted his account after a few month. Well, not to long after, someone new to the community, not knowing that the username belonged to someone with a poor reputation signed up with that username. Since it was a large community and that banned person had such a reputation, the new person was instantly overwhelmed by IMs and weird posts directed toward him!
It is much better to NOT EVER delete a member, especially a banned member.