Interesting thread/idea and lots of good comments here.
Yes, I would enjoy getting an occasional and spontaneous PM from a provider--particularly if it was someone I've seen before or have already initiated contact with in some fashion. For the ones I've seen before, it would be an indication to me that their experience with me was good (from their perspective). I would instinctively be more likely to go see them again.
On the other hand, I could imagine becoming annoyed by the "hey baby, I'm horny, come see me" messages which were obviously sent to a hundred or so other guys as well. As has already been stated, it's not too difficult to tell when someone has taken the time to compose something directed to a specific individual. Everyone's time is valuable, so when someone has clearly taken the time and effort to send a thoughtful message to you, well, that's really cool.