I think new users should be allowed to reply to PMs they received.
Imagine the following situation:
A freshly registered user posts a genuine question and received a PM about it from somebody. The new user cannot reply or even say thank you!
I can understand that this is frustrating and may lead to "post-count SPAM" - i.e. posting of various meaningless messages just to get the PM function unlocked. Such people are not SPAMmers, but genuine users!
Therefore, please enable replying to received messages for all users!