I figure they're sometimes helpful, sometimes not. It's a case by case thing. IF they're much wider than the tires, then they're probably bad, if they're too far back from the wheel, probably bad, hanging in the way of an aero shaped back bumper, probably bad.
I'd also figure that trimming them might be an option rather than removal, if they're fitted close to the wheel, and aren't disturbing any factory aero, then I'd trim them to about a quarter inch under tire width, and round the corners.
I remember The RoadWarrior..To understand who he was, you have to go back to another time..the world was powered by the black fuel & the desert sprouted great cities..Gone now, swept away..two mighty warrior tribes went to war & touched off a blaze which engulfed them all. Without fuel, they were nothing..thundering machines sputtered & stopped..Only those mobile enough to scavenge, brutal enough to pillage would survive. The gangs took over the highways, ready to wage war for a tank of juice
Some Saabs used a mudflap in front of their rear wheels. (Some older 900s I've seen.) And my old 9000 Aero has contoured deflectors right in front of the rear wheels (and a very odd little mud flap in behind the wheels.) If you see one take a look at them - Saabs 9000, 9-5s anyway, have a fairly low CD ratings I believe.