You're probably right and it will cut down on a lot of other unsolicited PM spam as well but I think the restriction/option to turn it on/off should be clearly stated so people are even aware of it. New users who run businesses here will likely want or need to receive PMs from them and they might be completely unaware of this restriction otherwise.
If a newbie is stopped from sending a PM due to this, they get get:
User _____ has not chosen to allow messages from newbies. You should post in their relevant thread to remind them to enable this setting.
And if you try to PM a newbie while you're ignoring newbies, you get:
User _____ is a newbie, but your options are set such that you cannot receive PMs from newbies. Therefore, you cannot send PMs to newbies, either.
So I
think that it should be rare for people to be unaware of it. If a newbie posts a trading thread, then they'll have people posting to remind them.