I guess I'm ambivalent about it as I have never really had any problems with people abusing PMs or spamming people. Lust lucky I guess. But if you are going to implement it, then why not make it flexible enough for both options? In other words, have an admin setting for each user that allows you to turn off PM send or PM receives capability independently, or both together. Radio buttons like this:

O – Disallow user from sending PMs
O – Disallow user from receiving PMs

Then you could select one or the other or both.

Basil

PS: As long as were on the subject of PMs, see my post about having a PM full warning for users so they know when their inbox is getting full