Seen from an energy point of view...
To get the engine spinning, you need energy.... and that energy comes from "stored energy" (in case of a dead engine), that stored energy will be the altitude below the aircraft, or the fuel in the tanks etc.
So no matter how you twist and turn it, if you have an engine windmilling, you're using energy to get that engine windmilling, that requires extra thrust on the other engine(s), hence extra fuel consumption.
I have to think a little more about the drag thing though, but I'm confident oggers has it nailed here.