There seem to be valid cases where a conversation isn't meant to be followed by the public.
The "Delta CS classes" has been brought up as an example. In that case, probably ill-advised users are creating posts that really aren't conversation starters, and which should have been posted on some blog or internal forum instead. It is a good thing, in my opinion, to not display these posts to some general public.
I can see the same point be made for a post that consists solely of an external link, without even a single sentence stating why that link should be opened, or what exactly a following conversation might be about.
It might be good to add a statement somewhere that "All" means "All posts that are actually useful to potentially start a public conversation", if that's the case.