ACTUALLY on front wheel drive cars the spoiler does 1 extra thing , it reduces rear bounce do to updrafts even @ speeds of 55+ at 65 mph the air going under the car is trying to lift the rear of the car so the rear spoiler is compensating for this. The rear ends of honda's are very light and have a tendancy for high speed bounce. With a real spoiler, this would be reduced by the constant air flow directed correctly, but as was stated before most of the spoilers made now are cosmetic. Which means no one ever puts them in a wind tunnel and tunes them if this was done you could get the optimal down force with minimal drag or resistance

so unless you can correctly tune a rear spoiler its cosmetic and also slows you down

take a look at porsche they now have the little wing thing that comes out depending on the speed, this has been tested on a wind tunnel and as you can see it is barely out @ 65 mph but @ 80+ it is fully extended