You will gain some HP and MPG with an electric fan. While its true that putting a load on the alternator will decrease it, when you are moving (above around 30 mph I believe) you are pushing enough air through the radiator that the fan is not needed and should not be running. Even with a perfectly functioning fan clutch there will alway be some drag from the mechanical fan.

Either mercades or bmw (I forget which) is using electric power steering pumps for the same reason now. While driving straight you dont need the pump running and creating a load on the engine so its turned off. When you start to turn the wheel it turns the pump on.