It would be confusing, seems like. After you've sent the PM, the other guy has that message. Editing it on your end isn't going to change what was originally sent, and editing/resending is just going to put another message into his inbox - no different than sending another PM.
It's like email, once you hit send, there is no going back. And you can be sure somebody would try to use the edit feature to reneg on a deal or try to retract an insult or something besides the convenience feature.