Update: since I posted the original message about why PM privileges were removed for newbies, we found various other abuses. For example, people hanging out for a while till they got the privileges and then using PMs to promote their various wares: insurance, real estate, etc.
We tightened up on our policy. There was a significant number of regulars who said that we'd be better off without PMs at all, so that might be on the cards too.
As it stands, however, people who show by their postings that they've read and understood how things work around here are more likely to have them than those who already cause us a lot of unnecessary work.