The way i see it, there is always "better build around the corner". Just like now, i´m sure FD have internal build(s) that internal testers are testing. They are not public (meaning Alpha atm) because they are not ready for public testing.What is the point of putting out a 2 week old, known buggy build if there is a new bug-fixed build just around the corner?
I see it in waves. When one build is ready internaly, they will release it to current Alpha backers. Why not release it to wider audience (Premium Beta for example)? Because it might not be ready for that audience. It´s easier for FD to manage testing if its first released to current not so wide Alpha group. When its ready, they will release it to Premium Beta. That doesn´t mean that there is nothing to test. Opposite of that. When most critical issues have been dealt with first, internal testing group, and then, Alpha backers, then build is much more ready to test wider issues in Premium Beta.
And hopefuly Premium Beta players/testers will get it ready for regular Beta.