I've just answered on a question if it worth having a forum on a shareware site. Yes. Here is why:
I have a forum and get the following benefits:
* The forum is used for support. I don't worry about support requests lost in the e-mail spam.
* I get SEO phrases and words from the real users.
* I get testimonials and don't worry if I can publish them on my site.
I use phpBB. Thanks to my bbAntiSpam tools, the site visitors can write without registering. Probably it's one of the reasons why the forum is alive.