After running a web forum for two years, and having just shut it down in favor of this blog, I probably have a unique perspective on forums on the Internet. Forums have their place on the web. They seem to be best when attached to a technical companies website as a way to support the companies products, especially with products like software or hardware that needs to be installed and configured. Inevitably, users want to know what each option does, or why they should pick one option over another, or what to do when their complex forum software won't run.
Having said that as a preface, now after two years, I'm opposed to forums on a web site. Why? They're not worth the trouble, especially for small businesses. The main problem is forum spammers. Forum spammers are constantly trying to put trash on your forum. Trash is ads for drugs, medicine, porn, or links to some other place on the web that doesn't belong in your forum. Surprisingly, there are few inappropriate comments to other's posts, mostly just spam with no reference to a subject or category in the forum.
Forum spammers make money by charging less than reputable companies fees for attempting to register for a spam account in forums. They use automated software to discover forums, and automatically register for an account on a forum. You prevent a registration one day, and the next day, their back with another name, or email address, or even url all done automatically.
What does that mean to the forum owner? Time wasted managing a forum. A forum owner who wants a clean well managed forum, now can not let anyone register for the forum. They need to require the person who wants to join a forum pass some sort of test to weed out forum spammers. Legitimate people who want to comment on your forum, just don't want to bother with an extended registration. It takes time to look at each registration and make a judgment as to what is a legitimate registrar and who is a forum spammer. You either make it difficult to register, or delete the new joiner who posts spam. Either way it takes time.
How bad a problem is this? I estimate that there is 1 legitimate registration to every 40 spammer registrations. Of course, this all depends on your site. That number may be high, but it's not far off. There are ways to block the 40, but as mentioned earlier, it takes time, and in the long run, the small business owner comes to the conclusion, its not worth it. Put a simple email contact form on your website, if the email is spam, you hit delete, and if legitimate, you have a one-on-one communication with your customer. Much less time consuming, and in the end, you help put these spammers out of business, after all, there's now one less forum for their registrations.