I don't think you need an entire forum, but you should have an efficient ticket system in the backend. Most good hosting companies have a way for a user to submit an issue or question via a ticket, and employees can see the unanswered tickets to answer them.
If you do use a forum, I would recommend making it a general hosting discussion forum if you want it to be active. If you want it to be used just for your clients then that's fine, but don't expect it to be busy with activity.
I would have to say that a support forum is not necessary, However if you have a large host lets say the size or arvixe, dreamhost or other large hosts it may be good mainly due to the fact that clients with the same company can help each other and lower the support ticket requests. But for a small provider a knowledge base should be sufficient. And The customer can always search online for web hosting forums to see if they can find help there.
I regret that why I use a forum. My google webmaster is flooded with errors related to my forum.
Recently I remove all links from my forum and my site reputation increased in seo checker site and even traffic increase from search engine so If you are a hosting company then do not use forum on your main domain.
- Do not use forum on your main hosting domain, If you want to use forum then host it on a separate domain like yourcompanynameForum.tld.
- Do not use your main domain for shared hosting name server. instead use different domain for NS.
