I don't think I would have even voted on this....not that it matters because I missed the cutoff anyways.
I'm on the fence and I'll explain both sides.
Going private can push more people to register, and help keep spam down...but at the same time it will KILL the sites page rank in Google. With any site, Google rankings are VERY important. I believe you can set forums to allow spiders to crawl a private forum to keep your page rank....but even then who is going to spend 10 minutes registering just to read something that may or may not help them.....when they could just click on the next link down (nasioc) that would have their answers.
Well I'm still thinking about this as I have spent tons of money to keep us online & growing. Our information is top notch, and now after 2 years our database is huge.