To be honest it doesn't really have an effect on not being able to pm, you would be beter off doing what is done on the MLR, and restricting the amount of posts a non member can post rather than not let non members send pm's
In all reality, all i have to do to get in contact with someone i want is start a thread and ask the guy to pm me with his mobile number, so not really a hindrance is it??
Just me 2 pence lads, restrict post count per month instead