Yes, I beleive a web host should have forums. It just makes it more of a community, and it is also an excellent way to have promotions.

My opinion is that if your forum is not active, its best not to have it. It looks bad to potential clients and may cost you a few sales. Either keep it active yourself by regularly updating it / emailing and trying to get people involved or just remove it all together.

do you want your hosting company spending the money you pay them on their forum or lowering their hosting prices and improving their uptime and support? we like to keep it simple and just offer 100% uptime and be so reliable that we have very few support tickets.

no one really bothers to even look at the webhost's website once hosting has been purchased...
i think you should mass e-mail every client saying you have a new forum and they must sign up for updates.