Scenario: a user registered successfully with a (temp.) valid e-mail address to get through validation. Then he posts SPAM/"bad posts" or links to illegal software. As a good admin we removed the post(s) and also the user from the forum. Later on the copyright/software owner contacts the forum admin (maybe found the violating post by a google archived search link and like user infos to track. But now the user info is not anymore available.
So an option "Archive user tracking info before delete" on the "Remove User" page would be of help. It could move the whole user record in question to a new table (same layout/columns as original users table) for later retrival.
Or move the topic to a hidden forum (like many forums have, eg a moderators secret forum for mods only to discuss things) then lock the user - that way it'll be hidden from view and search engines, plus you'll have the whole original posting for reference in these cases