The target for the logo is an option. It can either point to your home URL, or your main forums.
A lot of the comments regarding "home" and the logo are based on what other software does. If a forum is a component of your website, it makes sense to present it like that, rather than an island with a link back on the bottom right. Most people will use their own logo, probably the one for their site, so logically that would link back to the root of their site (as it usually does on most other sites).