A new user signs up. Because the number of members on my forum isn't yet too big, a lot of the common usernames are still available (bob, lisa, peter, jeremy ...). Unfortunately, these names are also associated with spammers in the StopForumSpam database, and therefore end up in the moderated queue. It can be hours before I get to the queue, and by that time legitimate may have lost interest.
I think I would prefer if this feature didn't consider usernames, only email and IP addresses, but there appears to be no option for this.