I meant to mention that -- Firefox uses ESC to exit fullscreen but Chrome uses Cmd+Shift+F. They need to standardize that.
But once you're full screen, F11 does indeed get you back to the original state. So I still don't see why ESC is needed.
The main reason is that you probably got into full-screen by pressing a button, not the F11 key, and now you just want a "get me out of this" key which escape is better for.
Although you would be breaking the standard convention.