I was running a beta server all the time and no one ever joined there.
Right, I remember one more problem regarding beta testing.
Not enough servers. Even if there were enough, there is a major problem when it comes to lag/ping, which can seriously cripple testing efficiency, especially if the servers had a ping cap. I noticed most of the beta test servers were all of a few same locations rather than spread out and people only play servers whereby the ping is below 300 for them.