Well, I personally don't really see a difference in the usability of notification center and control center, at least as far as what sort of background would be present. I (and I'm sure many others) use both in the middle of using other apps--that's part of their design, and in fact even more part of notification center design so that you could access notifications anywhere you are without interrupting what you are doing really--so from their functionality or usability or usefulness I really doubt that a pre-iOS 7 visual design would really change any of that for either one of them.
As far as usability, the overly white design with removal of selection or button outlines and use of light/pastel colors and thin fonts are all very much iOS 7 design elements that are more noticeable and more prevalent throughout the OS than the functional changes, and they are all some sacrifices on the side of usability (perhaps not to everyone, but certainly to more people than pre-iOS 7). Various changes to some apps like Calendar and Music for example, certainly have also changed their usability for the worse, at least to some degree for quite a few people.
Even, the very fact that Apple is bringing accessibility features to deal with more and more of these items (bold text, darken colors, reduce motion, increase contrast)--most of which didn't even really need to be there before--and making changes to bring back features that used to be present before--like some features in the Calendar app being brought back in iOS 7.1--is pretty much an indirect admission on Apple's part of all of that.
