In my experience, the problem with MM is exactly the same problem people face with the photography industry: since there is no universal governing body, there's no one to stop every jackwad (or I suppose "jillwad") from declaring themselves a model and trying to network.
The quality of the models I've seen varies GREATLY, as does the compensation. Just like photogs, there are tons of them who are more than happy to work on a quid pro quo basis, so they can build their portfolio. The photog has to bring good work to the table though, otherwise the model will expect payment.
I've never paid for a model through MM. There are more than enough very attractive girls with little to no experience who are thrilled with receiving a set of strong images for their book. My portrait work isn't something I advertise right now either, so I haven't done any of this in quite a while.
As far as the "trash". Umm...yeah. There are no shortage of skanky, skanky females on there. "Caveat emptor"...