Sad news if you were hoping to see fender skirts return to the automotive design mainstream; according to Automotive News, General Motors has already spent a fair chunk of time and money investigating whether or not adding the aero pieces could lead to increased fuel economy.
As it turns out, covering the rear fender wells could indeed go a long way toward making vehicles more aerodynamic, but the costs and drawbacks associated with applying the pieces are too steep for production. For starters, manufacturers would have to run narrower tires at the rear of the vehicle, possibly decreasing handling prowess at the same time.
In addition, GM found that running fender skirts causes tire pressures to increase dramatically, potentially leading to and increased likelihood of tire failure. That's not something that any automaker wants to induce in a production vehicle.
Then there's the fact that adding fender skirts to a vehicle would invariably add to production costs. At the end of the day, the domestic automaker found that fender skirts simply aren't worth any additional aerodynamic advantage given the risks and costs associated with their use. Oh, and most consumers don't really seem to like them, either...