I'm a professional tester (just not for games), and I can tell you that there is a WORLD of difference between user acceptance testing (equivalent to beta testing) and hiring pros. This is not to say that you can't succeed with only one, or that you're doomed if you pick one over the other...
1.) Pros are generally able to get involved earlier, partially since when you're paying someone, and they advise you to do it, you listen better than when someone you got for free tells you to do it. A rule of thumb in the software industry is that any bug costs more to fix later in the cycle than earlier. Also, by the time you've gone to beta, marketing has set a release date and wants it OUT THE DOOR.
2.) Pros understand common things to test. Users often don't. That said, betas/users have a better understanding of expectations, and will bring up things that would never have occurred to a pro.
3.) Pros will have automation tools, automation testing experience, and the like. Automation tools can tell you more than "it's slow" - often it can give you insight as to why it's slow, before the devs even have to pore over the code. Automation also lets you run many more sessions than betas possibly could. While the devs could also automate it, it's simply a different mindset between functional testers (manual), automated testers, and devs. Having all 3 types of testers is optimal from a quality/time perspective, if not always cost.
In a perfect world, Paradox would do both. However, that means they're spending extra money and time having someone deal with the goat rodeo that is a beta. Being in the middle of User Acceptance hell where I work now, I can understand completely why Paradox didn't want to do both.
Oh wait, I'm in Integration, System, and User Acceptance Hell. Three hells for the price of one!
