I do apologize in advance but I have never seen the point of this. If someone visits a forum, while not being 'invisible', then goes 'invisible', they are listed on that page for 24 hours with no way to delete the information.
On our site, it would only discourage people from being visible because some people would not want their visit recorded for a variety of personal reasons.
If I wanted to make my site look more busy I would turn up the time online from 15 minutes to something greater.