We pulled into Whitehorse in the Yukon after driving extremely slow on a gravel road. In front of us were two motorhomes also driving slow. They had the rear flap and we had only a flap behind each tire. At the RV park they were removing the rear flap. They showed us a layer of 1/2" stones at the base of their windshield and many marks on the hood of their towed vehicles. Our Jeep had absolutely no damage.
Later we were driving through Salt Lake City on a very rainy day. I began watching the opposing traffic. With the motorhomes with the stiff flap across the rear, the water was being thrown in a high arc onto the hood area. Those without a flap had the water thrown at tire level.
That convinced us that we were wise in not paying extra for that advertisement on the rear.