I personally don't mind people catching other people in a PM. Sometimes, in the right circumstances, you can use it to your advantage (I won't say how ) It's just kinda bummy when people use that as the *sole* reason to suspect other people.
You see somebody in a PM, go back to their posts and analyze that IN CONJUCTION with what you've just seen. It will make you a way better player in the long-run and won't derail discussion into lazy accusations.
I've said this in the past with my games before and there WAS agreement from players to approach the game this way. However, if it is easier for everybody to just be on anonymous, I'll switch over.