If the flap gets mashed up toward the track when backing, the support plate gets bent. Once that support plate is bent, the flap will get sucked in. Most guys bend this the first time they have to back off a trailer.
If you bend the support plate back up it helps. But, now it is physically weakened and in bends in that much easier. So you end up bending it up much higher than it ever was to keep it from being pushed in again. Then, with it this high, your hozing whoever is behind you. If you are on a gravel road, you can damage their hood, windshield and possibly even put stones in their face. So cutting or bending up excessively is not a good answer. Also, for the same reasons, shorter flaps are not so hot either.
Also, if you have a shorter or mispositioned flap, cooling is affected. If you can't keep some snow circulating' up around in there, the sled may overheat.
So, if you are not going to be careful when backing, the straps appear to most folks as the best answer. I have done that on the sleds the kids drive around.