To enjoy your desktop picture in full splendor!
Seriously, there have been time when I wanted to hide my dock and menubar, and just make the screen my desktop picture. I suppose it appeals to neat-freaks.
I think this might be difficult to impliment, because, as you can see, if you force-quit an app, it's menubar will just go away, until you switch apps. It's like each app has it's own menubar.
But, hey, if someone finds a way, please share...