Dead DPS does zero DPS. We all know that saying. I introduce to you my Princess Bride collorary to the Dead DPS rule:
The perfect description of what happens when a mob dies in an RPG aside, Miracle Max is right: mostly dead is slightly alive, and slightly alive means you can still do more damage. Naturally -- since we are magnificent, resilient bastards instead of glass cannons -- warlocks are quite excellent at staying slightly alive.
Miracle Max: Whoo-hoo-hoo, look who knows so much. It just so happens that your friend here is only MOSTLY dead. There's a big difference between mostly dead and all dead. Mostly dead is slightly alive. With all dead, well, with all dead there's usually only one thing you can do.
Inigo Montoya: What's that?
Miracle Max: Go through his clothes and look for loose change.