Recently there was a private beta site, which I had not been involved, but I successfully created an account and accessed all the content of the site. I remember private beta sites were used to be close to outsides, but why could I make it?
It occurred to us that it didn't really make sense that you could participate in a private beta if you committed on Area 51 anytime up to the launch of the private beta, but not once it started.
We still want private betas to be limited to users who are actively interested in building a site (vs. lurkers or drive-bys,) so we're not letting anyone with the site url or who googles straight in.
But if you are willing to put in roughly the same effort in navigating to the Area 51 page that would have gotten you access as of the day before the private beta launched, it seemed silly to lock you out for a week. So, we stopped.