I'll try to make myself clear with an example.
Imagine I had read Tomo's books (as I actually have) and would like to discuss something contained in there given my quite peculiar background (this time I am making it up: I still haven't pondered enough about the books). As far as I know, at the moment, I have only two options:
1)I can PM Tomo, but this would severely restrict the discussion cutting away other people who could be interested.
2)I can wait until I escalate privileges in the forum and get access to the "very restricted areas", but of course this could easily take years and not happen at all.
So I am thinking that if could write a post in the common section of the forum, but restrict the access to the post to the people of the Naked Mentalism area (and myself, of course

There could also be a dedicated subforum in the general are dedicated to restricted threads...