One more reason not to depend on forums alone : The top-tier advertising networks like ValueClickMedia do not accept forums. They accept only if your site is a portal (The site can have a forum as part of it). So don't keep only AdSense in mind. Just in case, AdSense bans you in future, then you cannot apply for many other advertising networks if your site is just a forum. So always use a CMS / Portal that has built in forum (or integrable with forum like IPB / vB)