You can't release iOS7 fixes "in the meantime" because the SDK won't allow you to submit. So it becomes very difficult to chase and fix bugs or test the failures, even if you can test the failure on the 7 SDK and go back to the 6 SDK to test and submit -- you can see the problems that could arise from that. It would almost be like developing for Android! (only joking..)
Beta software is exactly that -- beta. Users should expect failures and problems, so giving any sort of review based on iOS7 beta software is absolutely ridiculous. The funny part is that the only people who should have the beta, should know better. But obviously they don't. I wish there was a way to lock down access to the beta program to legitimate developers, but that's impossible because it wouldn't necessarily be a fair system. Although it could be possible to restrict beta access to developers who actually have a released app in the store. New devs could still use the existing SDK and tools and submit, but they wouldn't necessarily have access to the betas. I don't know what problem that would solve, however. It doesn't seem to be a significant issue, except in app store reviews, so the simple solution is to restrict reviews to non-beta people and instead of reviews, iOS7 users can submit private bug reports. This way devs are aware of problems they might not know about but it doesn't give the negative publicity that affects their sales. Several one-star reviews can kill a newer app before it even gets off the ground.
One potentially cool alternative is to allow devs to release iOS7 beta versions that are only accessibly to iOS7 beta users through the app store. However just as promo-code apps can't be reviewed, the same logic should apply to iOS 7 beta users.