It helps cut down on the automated spammers, one post wonders, and home work due tomorrows. It's not perfect, no matter what is tried, someone will try a work around. Maybe restrict posting to a single, or a couple forums. But I don't think it's a better idea than what we have. New users are important to the health of the site, just a few are the problem (likely the same people, different E-mail/logins). It's important to welcome new people, and get them involved in discussions, which means they need to be able to reply to the threads that interest them most. Nobody likes it when everybody must suffer, just because of a few bad apples. We do need to control spam, as it can make the site useless.
I don't what all the current restrictions are on new users, but think keeping them to a minimal inconvenience is the best strategy, since we need the new people to keep the site strong. I believe there is an option to 'flag' a post, and you give a reason for being offended. This would bring it to a moderator's attention, who would take appropriate action. They do a great job here, and it would help out some.