I understand the reasoning behind this limitation, but I don't see why it would be effective. If some spammer wants to spam PMs, what's going to stop him spamming 10 posts as well?
However, the biggest place it is a problem is in threads like Looking for RaF/SoR, so I'm thinking why not at least let people under 10 posts reply to PMs, ie. you can only send a PM to somebody who has PMed you first. That way people can communicate with keyboard turners at their own discretion.