Turning it off is an option however I'm unsure if this also turns off any automated 'sendnotice.php' email notifications.
Turning the Message System off also leaves access to the user's Classified Profile information, which although good for Classifieds Sales 'Feedback' has gotten a number of my users confused, because as well has having three Private Message Systems on my site I also have three Profile/biography pages because I'm using pp_Classifieds and pp_Pro on top of vBulletin.
The pp_Classifieds I need to retain as vB
has no in-house alternative at present, but my pp_pro (Gallery) will probably get replaced with vB
's own solution if I can't redirect the pp_Pro PM's and mask the Profile pages.