Do forums help a business? That is a tough question, and the answer would have to be sometimes. Depending on which state your hosting company is at, you may or may not need one. Would your forum be for support purposes only?
I think that the hosting companies that start up earlier are more inclined to have a forum, this is because they do not have many customers and they need to look like they are always active. Not saying they aren't, its just that they need to show exactly what their service is about. The problem with new companies haveing forums are that there will not be very many users, and that may make the company look bad. So overall, it really depends how you look at it.
The larger companies do not really require a forum because at that state they should have a good line of support options. There will also be very many customers, so a forum might get a little messy because of the fact that the members would be on it for support- sometimes frusterated, they may not want to talk about interesting sites (example) in the general board...
If you are thinking about having a forum for your company, make sure you look at all of the pros and cons that would relate to your specific needs. Good Luck with whichever you choose.