The problem here is that I don't know who I want to exchange PMs with until I need to. Then I have to follow, hope they get the notification that they've been followed and hope that they follow back.
The other problem is that if you follow me, first I'm not going to know since I don't have Inbox emails enabled and second, even if I do discover it, I'm not going to follow back no matter who you are because I just don't want to get a notification, here or via email, about all that anyone here does. No offense to anyone, there's just little value in knowing all that another user does as far as I'm concerned. I suspect lots of others feel the same way (some have spoken up here).
With several hoops to jump through for both parties, that effectively means there is no useful PM system here.