This is a good point, but doesn't seem to be like a very big one. (I'd like to be able to turn off my tablet display when I'm not using it.)
It seems to me that Apple's emphasis has been on using one screen well and applications like FCPX that use privileged APIs to use fullscreen well across monitors, but developers who use multiple displays in more complicated ways are pretty much stuck with third party solutions (I use SizeUp, and I used to use Cinch). This doesn't solve the issue you mention, but I'm very happy with it otherwise -- and when Apple incorporates the features of nice third-party utilities into the OS it gets lambasted from the other side.
It seems to me that Apple's emphasis has been on using one screen well and applications like FCPX that use privileged APIs to use fullscreen well across monitors, but developers who use multiple displays in more complicated ways are pretty much stuck with third party solutions (I use SizeUp, and I used to use Cinch). This doesn't solve the issue you mention, but I'm very happy with it otherwise -- and when Apple incorporates the features of nice third-party utilities into the OS it gets lambasted from the other side.