Well, if you're trying to ignore a member, then invisible mode not only keeps them from knowing when you're really on your IM program in invisible mode, but in the event that PM tracking was enabled, people would not be able to see the last time their PM was ignored. Obviously, if you post, you've blown your cover, but some people are simply compelled to visit; posting is optional. In case anyone is wondering, the today bar for invisible users does not register the visit until the first post for the day is made. (Though it is still counted toward the activity bar).