I use PMs
1) to talk to someone about something that I don't think wouldn't interest anyone else and therefore avoids cluttering up the Forum with boring posts.
2)to say something that I don't want to be picked up from a post by someone doing a Google search. I got "burned" some years ago when someone in Greece took exception to my lighthearted comments on a particular place that they construed as being critical of their (unnamed) establishment, and tried to claim compensation for loss of business. Since then I have not named the particular place or any of the inhabitants in a thread but have in PMs or e-mails to interested members.
In the case of (1), if the recipient felt our discussion of wider interest I wouldn't object to it being aired more widely. In the case of (2)wider publicity would defeat the point of the PM.
I don't carry out arguments or disagreements with individual Mudcatters in public or by PM. Life's too short to niggle and whinge. We disagree? Tough, move on, get a life.

RtS