I really depends of the type of business. A company that sells products, tangibles, may not be interested on keeping a forum that may be overloaded with support questions and maybe blaming the retail for not providing answers.
Notice many ecommerce sites have the option to add reviews about the products, that is as much the seller could be involved safely without getting into having to monitor forums and start giving support that is not part of the services of the business.'
Now if the business offers services such as hosting, a forum could be a very good thing, having the option to easily provide support to their clients.