Yeah, I see what ur saying and all, but I still would think it benefits a RWD a hell of alot more than a FWD car. A FWD car, similar to lugging a trailor (for one extreme), is basically being pulled so ur back end is more inclined to stay in which ever direction the front wheels are pointing. With a RWD car, the rear wheels are pushing the car, like a shopping cart (for another extreme), so the car is more inclined to go in the direction of those wheels; a straight line usually, but not if ur wheels get loose and cause fish-tailing (which isn't an issue with FWD). So with my view on things, the wing keeps that rear planted so it won't fish-tail. All at speeds above 40-50mph of course.
If anything a wing on a FWD is for a lazy driver that can't keep his steering wheel straight, lol. Whatever the case, I still don't think it's necessary on a Civic or the like... juss something to spend money on and obscure ur rear view. It also doesn't usually help the looks any.