rear spoilers on new cars are simply purty ups --like much of the pick up truck "trick" stuff.
As you have likely seen in tunnel test - if'n you want the car to stick n go fast --- then clean up the belly drag and pressure problems - but you can't sell that idea to the typical buyer -
For his time - Glenn Curtiss was simply brilliant -coming up with the movable panels .
(I think it was his idea )