It appears that a lot of people have trouble telling the difference between the words personal and private.
Actually I may be wrong looking at past events.
Looking back actually it seems yes they should be kept private.
Huh.
Anyway, I don't think anyone has a reasonable expectation that their PMs are going to be kept private. Some members think they're private conversations while others definitely don't--and I've posted quotes from PMs before myself, but in my case it all depends on how "private" I thought the PM was in the first place. And if someone asked me not to repeat what was sent to me, I certainly wouldn't, but I don't think that's happened yet.
And if the community frowns on that practice, I would respect that. I'm not sure there's been any consensus on the matter, despite a few opinions being expressed here and there.