There might be some confusion if there are a large number of Spammers joining the forum, and then getting subsequently banned and removed. That's just the first thing that comes to mind for me.
Spambots often manage to create a new account but fail at activating it (to simplify crudely) - I'd guess a lot of those users never had their accounts activated. Last time I checked, SMF had a feature for pruning unactivated accounts after a set period...
Of course, sometimes you can get flooded without having an actual Viagra/iPhone spammer, but...
