My guess is most things said in PM's could be said in public chat, or not at all. I never send model's PM's. A turn off for me is when a model stops what she's doing and starts replying to all the PM's she has.
Here's my suggestion. It's ok to charge for friends, but, have these friends send MFC mail instead of PM's. When the model goes off cam, she can respond to the MFC mail, and ignore those sent from non-friends. I once felt a need to tell a model something and tried to send a PM. I found I couldn't. I noticed from her chat room who her regulars were and sent one a PM and he could get the message to her. Essentially, she was angry with a lot people who were trolling her so stopped reading the chat. She had banned several people already and I just wanted to pass on she could consider extending those bans to 60 days after she logged off and return next time in better control of her room.
If a model says PM me before private and wants to block PM's, well all I can say is they will have to figure out ways around that.