Way to ruin the debate by quoting forum rules, squeakers! (I kid.)
I think the key point is "try to resolve." Sending someone a nasty or argumentative PM is not indicative of someone trying to resolve the situation. Sending someone a PM that says, "Look, it seems like we get on each other's nerves a lot. Is there something we can do to make the forum more pleasant for each other?" could be construed as an adult way to handle a disagreement, which should be done via PM and not on the thread itself. In my interpretation, of course.
I'm naive, I guess. My PMs have all been friendly exchanges. I'd respect someone trying to resolve a difference, or further explain what they were saying, but I'm a little surprised that people send nasty stuff via PM.