Not sure I'd agree to that. The new forum would need to feature a spam filter and a spam folder, so the PM interface suddenly becomes an email interface ("hey I've written you an email, why do you keep answering me via PM?") and newbitcointalk becomes an email provider, with people signing up just to have another email address.
Yes, I do agree that there would have to be substantial checks in place to prevent spam and abuse - but it's nothing a third party service provider couldn't handle. As for people signing up JUST for an email address - that begs the question, is the new forum going to require a proper email address and verify it? If so, there are many other free email providers that don't require any verification.