I'd leave them on with a very clean, unmodified original car, but whip them off if not. They tend to look scruffy if not kept clean. I've left them on the 360 but they are off on the 340 daily.
One advantage though is, as you'd expect, they keep the mud (and stone chips) off the rear lower wing and out of the bumper
The front mud flaps are even more important at stopping the millions of small stone chips and tar spots you get off the front wheels. These end up hitting the leading edge of the rear wheel arch.









