As someone who's been on a few NDA'd betas before... I just don't understand why people leak. Is it the feeling they can break the rules without likely facing truly serious repercussions? (Has anyone ever gone to jail or even been seriously fined for violating a video game NDA?) Is it the notoriety of it?
I mean, I won't deny there was a fun factor to having a New York Times reporter beg me to violate a Google NDA once. But even then, I had more fun knowing something the New York Times didn't than breaking my agreement, ruining my personal reputation/credibility as a trustworthy individual, etc.
And generally if I have access to these sorts of things, I'm enthusiastic about what the company is doing, and the last thing I'd want to do is sabotage them.