The StackOverflow software (if it has a generic name by now, I'd like to know) is fairly mature by now, and most of the kinks have been worked out. What's the point of having a beta period for SuperUser? It's not like you're introducing something radically new -- just a new topic.
The SuperUser beta period is more for the moderators and the community to flesh out what is going to be acceptable and unacceptable behavior more than it is testing out the software itself. It is important to get this nailed down so that the guidelines are going to be something the entire community can follow going forward.
There is also work being done tweaking the UI (stylesheets, etc.) on SuperUser.
I see 2 points:
- Avoid the "empty restaurant" effect and...
- Make sure there is some (good) guidance/community rules going before it's open to the public
The beta period allows for questions and answers to be generated before it is open to the "general public."