The problem is that the forum script requires that an administrator manage accounts. The moderators can remove posts, but they can't zap spamming users. So if a spammy user starts posting garbage all over the forums, the moderators can remove the posts and alert me. They can't shut down the user, however.

This goes for approving accounts as well. If a new users requested to join, I would need to approve it. None of the other moderators could because they wouldn't have the appropriate permission levels.