I don't have a bicycle with a derailleur, so I may be wrong, but don't you have to stop pedalling when you slide over to the next gear, or at least slack off? You'd have to be modulating tour throttle all the time. Also, on a moped, when you let off the throttle, you get braking from the engine, which stresses the lower length of the chain, and I don't think the derailleur could handle that well.
Besides, a moped engine will only get a bike up to a certain speed, depending on its power. Higher gears won't give you more speed, just lower revs at that speed. It might help acceleration, though.
If I were going to put "gears" on a moped, I would use a Nuvinci hub: