It also allows those testers a unique and better understanding of the game then almost anyone else. That could mean the difference of playing this game 3 months before it's release, or a year or more before it's release. So instead of being able to help shape and mold the game for months at a time, you are left out in the cold until they just need a few thousand warm bodies to stress test the servers a month or so prior to release. I've seen the people you're talking about. I remember one beta tester in Allegiance would write up detailed reports.
Companies are lucky if they get testers like that, though.
Beta testing is not as much as finding out bugs in the game and issues with gameplay as it is two test two important things. For games with servers, it load tests them. At least, that's how I see most beta testings to be conducted. The people who didn't like the game, mainly because of beta bugs, tend to yell louder than the people who would hype it. But I consider finding bugs fun.