It's a simple messaging device that works well, but has few user controls. Most particularly, while you as a user can block notifications from another user, you cannot block another user here from sending you a private message. And, you cannot delete a message from another user without opening it first.
This matters in discussions that get so heated that arguments flow over into private messages where it can get pretty ugly - threats and such. It would be very useful for the messages to allow a user to block someone from perceived harassment, but alas we would have to alter the code to make that.
Also, admins on the site do have the power to read private messages. That means if you get a message from someone you regard as threatening or otherwise seriously inappropriate, and you contact us admins to do something about it, we cannot use, as a reason to not get involved, that we can't read the message. We can.
As a default, we don't want to get involved in private matters when it really should be worked out among the specific individuals. Still, we have legal and social responsibility for the site so we can't ignore everything.
One solution is to allow private messages only for those who reach high trust levels on the system. That alleviates much of the potential for problems that arise from stepping into personal arguments.
But private messages here can be useful for community building purposes if you do not know the personal email address of someone you wish to contact and you don't want it publicly declared.
So, what to do?