I know of a forum like that.
Think I read somewhere that some forums do this for a physcological (sp) reason, the idea being that you register out of curiosity then, hopefully, like it and stay/re-visit. Bit like going to ASDA and they've moved the stock around so you have to go further into the store to find what you are looking for. Whilst you are looking for what you want you spot something else that catches your eye and browse a bit more.
What have you got to lose by registering? There may be something there that attracts your attention and if there isn't that visit, and sometimes there isn't, then its much easier to pop back at a later date and have another look. Forums ebb and flow as I was trying to point out in a thread the other day. Unfortunately some begin to ebb and seem to forget to flow.
If it appears to be a fledgling site then what have you got to lose by getting in on the ground floor and possibly making it a better site?
You are not restricted by anyone anywhere on how many, what type/how often you browse/participate in other forums.
As Nickso says, name the site, I'll have a look and then you will get at least 2 opinions before you sign up and make your own choice as to visit it again or not.