The high end coach manufacturers don't put those full width flaps on their coaches because they are functional in any way. They put them there because they have the manufacturers name on them, and they are perfectly happy to let the owner pay extra for advertising their product.
Most after market installations, in my humble opinion, are done simply because the MH owner thinks it makes his rig look better.
I saw many different techniques tried, on our trip to Alaska, to protect the toad and the rear of the coach. The protect-A-Tow (in my opinion) did the best job, of accomplishing both tasks, by far.
I've been looking for a full width flap to put on my old Bounder, but it would have to say "Does This Skirt Make My Butt Look Big" across it.