I read this: http://buddypress.org/community/groups/creating-extending/forum/topic/the-importance-of-private-hidden-groups-forums-activity-in-directories/#post-50337.
Has anything changed? More importantly, on a related note (but not exactly the same trac ticket #2166) I would really like for forums to display all forums the logged in user has access to, not just forums from public groups. Otherwise, what’s the point? Per @apeatling‘s comment on the trac ticket “Yes, but this will need to happen in a future version, there isn’t this level of privacy control in BuddyPress yet.”, is a logged-in-user-aware forum view also not available until either BP1.3 or maybe something with @jeffsayre‘s plugin?