The only beta of Mac OS X I've used was the Mac OS X 10.0.0 Public Beta. I think it was available for $9.99, and it had a note on the back that said something like, "Thanks for helping us make the world's most advanced desktop operating system." It was amazing, though extremely slow.
While I think that having public betas again could help, I think the other issue is whether Apple even listens to the feedback they get now. If they're not listening now, getting a lot more feedback won't help. In my case I have been a registered developer with Apple for many years (way before the official iOS and Mac developer programs), and I still have my log-in credentials in place; however, I have never been able to log in to bugreport.apple.com, which is odd because my Apple ID works everywhere else. It's not that I receive an error that I am not allowed to log-in; it's just that nothing happens when I try logging in. I e-mailed and called Apple's developer support and they have never responded to me. I'm not sure if they are more responsive to paid members of the development programs. But I also notice they don't even follow up on more general feedback for their web-site. If you look at apple.com/science, you can see they are still advertising Leopard and Mac Pros that are two generations old. I submitted feedback about this that was never acted on. I also submitted feedback on Apple's grammatical errors in iTunes and on their site that was never acted on (Apple never uses direct address commas, e.g., "Welcome Mark," and they often use excessive commas where they're not needed). The only feedback I've ever submitted that may have been acted on was when I sent feedback regarding the MacBook Air page that listed it having the wrong graphics card. However, it's likely they noticed it internally and never looked at my feedback.
Anyhow, my point is that Apple may not be a company that responds to feedback well regardless of how much it gets. I've been trying to get them to fix Bluetooth File Exchange in Lion and at version 10.7.4, it's still not fixed, and I'm not even expecting it to be fixed in Mountain Lion.