Hosting for posts can work depending on what your forum is about. I know one guy, who's running a gaming forum. So he hosts people and long as they post on his gaming forum. How does that help him? I suppose it makes his forum more active and since he's already dishing out money for a reseller/dedicated server, it doesn't really matter, since he has to pay for it reguardless of anyone using the space or not.

I think it can be a good business, but it's very risky and tricky. Many hosts that do try this technique, usually will fail at some point, though not ALL have. Personally, I think you should just stick to charging with money, rather than posts.