What does a beta tester do... Well, a beta tester gets the most actual build of the game and then gives it a proper beating to see if everything works as intended: 3D models, general features, the editor, animations, unit behavior etc. If everything goes smoothly there aren't any game-breaking bugs and everything works like a charm.
The flaws... There is a pretty limited amount of beta testers and some things simply slip through the net because you can't possibly cover every situation that might occur during gameplay. The bigger the number of people actually playing, the bigger the chance certain flaws and/or bugs become obvious. That's the reason even AAA titles need patches - because with a limited amount of time and a limited amount of testers not every angle can be covered.
Although I think you refer more to certain things not quite working the way we players want it to. Weeell... I can give you a REALLY long list of answers Steve gives to people every time they complain about something. "Not feasible", "too time-consuming", "not necessary", "planned for a later date" are the most common ones. Maybe the game is in exactly the state he wants it to be. Maybe it is in a pretty sorry state actually but he can't do better with the resources at hand. I don't know.
And: Yes, there certainly are some that simply want to get a shot at the game before everyone else.