There's a really good reason for wanting to disable fullscreen mode for me. I use DVD Player, a lot. When an app enters the "Apple" fullscreen mode, it switches back and forth as if it were a Space, but the problem is when CMD-TAB switching the window that was open previously in the Fullscreen app is not restored. Until you press the key combo for fullscreen mode again.
In DVD player particularly this causes tremendous problems resuming playback if for any reason you need to switch out while in fullscreen mode. I didn't have this problem in Snow Leopard as it was seamless - CMD-TAB back and forth no worries. Now if I want to switch back to DVD player and continue watching my movie, I have to hit CMD-F once to make the app come out of fullscreen mode (the screen does not change even though the app is supposedly out of fullscreen mode), and then I have to hit CMD-F again to finally get back to my movie, and even then it's glitchy.
This is why I hate Fullscreen mode as it is in Lion and later. Is there honestly no way to just kill it completely? It's very frustrating having Apple shove things down my throat when they don't even work right.