I actually think the forums should be viewable by the public, if only from an SEO standpoint. Once the spiders can see the actual content, the forums will turn up in many more Google searches (because more content is accessible), more people will view the site, and more people will join. Simple as that. Plus a lot of people don't actually join to see some info as people say, they often just leave.
Arnies is currently okay on SEO, but I'm sure it would shoot through the roof with public forums. There's no reason some of the most well-respected forums should be lagging behind some other