Suppose you're running a full-screen application on your iBook, say... a game.
Now, suppose you don't have your iBook connected to the electricity and you run out of battery.
Computer goes to sleep.
You fetch the power adaptor and plug it in.
Computer comes back from sleep.
Then you see your full-screen application again, but nothing seems to work!
Keyboard is dead. Mouse is dead.
A sudden flash in the screen and that single frame is enough for you to figure out what is happening, while you recall that moment when you ticked the "Require password to wake this computer from sleep or screen saver" option.
So your full-screen application is on top, but there's something else, that you really can't see except when the screen flickers, asking you for a password and not allowing anything else to work until you provide it.
And, since nothing actually works, there's really no way of selecting that box where you have to insert your password...
I managed to get in control of my computer again without having to shut it down, but I tell you, it wasn't easy, and it surely wasn't pretty.