The solution of this issue of Private Forums seems to me very simple. At least, in theory. I don't know if it could be made to work in practice, but I don't see why not.
The problem is clear. Some posters hijack or derail threads to the detriment of genuine discussion. As DB put it:
"I suppose one chief difference between private forums and public is that when those who ask questions in private forums do so, they are truly interested in an answer, in learning something. If the answer is unclear, there is a back-and-forth until everyone is satisfied. On a public forum, however, the intent of questions is often not to learn something new or different but to set the stage for expressing one's own view."
My solution would be for those who want Private Forums for the reasons DB has outlined should have them but they should be viewable by all t2w members in exactly the same way as any other forum. However, only members of the specific Private Forum could post. Membership would be determined by the 'owner' of the forum, just as it is now. If someone applied to join, and was accepted, but then posted in a way that disrupted discussions, then his or her membership could be withdrawn. If someone who was known to have a disruptive influence applied to join, and wasn't accepted, I don't see that he or she could have any cause for complaint.
In other words, matters would be organised as they are now, but in public view. The only possible objection to this would be if what was being discussed on a Private Forum alluded disparagingly to non members, or was somehow 'exclusive' to members, neither of which (I assume) should be the case.
I do think the current fashion of Private Forums operating behind closed doors is overly clubbish for an online community which has always seemed so welcomingly open. Apart from anything else, how could anyone even know if they might be interested in joining one of these Private Forums if they haven't been able to participate as a 'fly on the wall'?
All the best things that have happened in my life have been the result of wandering through the world with my eyes and ears open and my mouth shut. Walking past firmly closed doors always makes me wonder if I might be missing something. Obviously, privacy is important, but surely not on t2w, where few people are themselves, anyway?
Dod