I'll throw in a simple (maybe too simple) but straight-forward use:
You have an animated (cartoon, maybe) detective story where the bad guy is lurking in shadow near your good guy detective. You want the detective and most of the scene well lit without harsh directional lighting, but with the bad guy shrouded in shadow, even though both are in the same room at close proximity.
One quick solution: Put them both in a well lit room and then add a strong but short range negative light to create the gloom that hides the bad guy.
It's more artistic lighting than realistic (and one darned cheesy example
), but having a negative light capability makes it easy.
rj