I know many hosting plans are available in many hosting business.
Me, being a hosting reseller never tried those things.
I just want to know why they have such plans, and How much they earn from that.
Do you have such plans to give your clients for your customers ?
Is this an effective way of attracting people ?

I dont think so its a good idea. Its already common for this idea and I would say many had failed. I myself, had once been into this offer, but the hosts I was on failed a few months or years later.

lifelesspeople.com has been around for a while now. Proof I guess that this can be bottomline positive.

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.

But I know many sites provide very good service and they really have a very good uptime(Luckily my clients dont see this). But the thing is if you fail to post in their forum, then your site shows an error message:"Login to the forum and post" , Which is really an embarassing situation for any user. I think that should be avoided.

I've never seen anyone with a "Login to the forum and post" error. I've seen hosts put up a suspended page, but not anything like the one mentioned. Then again, you get what you pay for! Free hosts take off all ads etc.. if you pay them a small fee.