Spaces, aka. Mission Control, aka. "whateverthehellthey'recallingitthisweek", is meant to multimonitor set-ups with a bunch of apps in preset places. Use spaces/missioncontrol.
Personally, I don't get full-screen apps at all and wish this feature would die a painful and horrible death. I'd love to see stats on its use, as I've never seen anyone who actually knows what its cryptic icon represents.
But it is total garbage on a large screen, so I don't use it when docked.
My common use case is opening text editor in fullscreen on main display and terminal with tests and tail -f log on the secondary one. Using Lion's fullscreen mode could help me get some extra screen estate for the editor, were it done right.
Also, why force turning off the displays? After all, one can press the power button or dim it using brightness settings, if the need arises.
I never use it on my 13" MBP, whether it's just the laptop or if I have it set up with an external 24" as a second monitor.
On my 27" iMac the only time I have a window maximized is when I have iPhoto open. However, the window is maximized, I'm not in fullscreen mode.