I'm not sure which forums Ginger has set to "members" only, though I think it is significant. We always leave the homepage available, and usually topic or thread titles. People who search for specific things will find through the titles a hint of what they are looking for and hopefully are encouraged to register and join the site.
Guests can also be "bots" -- Google and other search engines are constantly crawling the site for new topics that you all post, which gets indexed in the search engines and that also brings new people to us.
You want to keep a blend of private and public areas -- make it public and we will get overwhelmed with spammers, trolls and bad elements. Make it too closed and people won't participate.
It's a fine line.
It is all Ginger's call as to which areas are more private than others.
