I noticed that we get a lot of guests checking out our site way more than the actual users.
I for one only log in if I want to post. If I just want to read posts, I do it as a guest. I am sure that others do this too. Why log in to read if you don't have to?
The software undoubtedly allows admins to change the settings so that one must log in to read posts. It could probably be set so that guests could view thread titles, which could further whet their appetite.
Or some forum sections could be read by guests and others not. (There could even be private sections that would be invisible to non-logged-in members, if that was desired.)
There may be people who only lurk as guests and never register because they never post, so if you did this there would probably be more registrations.
I'm only pointing out these possibilities in case anyone feels it is a problem. The status quo could be fine.