If they leave because they can't make money posting then they never really cared about Bitcoin anyway. These people wouldn't care if this was an auto parts forum. They would just run around and post about alternators. I can see only one good reason to allow it. The increased site traffic from these people makes the forums advertisement space more valuable.
I don't care about those that are here only for the "free" BTC, but there are quite a few users with ad signatures whose posts aren't spammy.
The increased site traffic from these people makes the forums advertisement space more valuable.
I'm not so sure. Bitcoin companies can promote their services by winning forum ad slot auctions and by paying people to post with ads in their signatures. Without signature advertising they will only be left with forum ads, which would drive the prices up.