That is a leap in counter intuitiveness, eh? Aluminum is too soft so lets put something 50 times softer over it so it will slide better? I hear, I see it, but I don't buy it.
As far as aluminum sliding easier than steel over sharp rocks, that is too subjective to define that easily.
See all those marks on that skid plate? Notice how they go from front to back? If one was to operate from the position that aluminum gouges and hangs you up, it would seem that it would be very easy to examine a skid such as that one and make a very definitive argument based on the number of marks that stop a short distance in with a resultant gouge at the end of each that stopped forward or rearward progress.
There is a reason you don't see them because for all of those marks, not once in hundreds of trails in JV has that ever happened.
I did get hung up once on my gas tank skid, but my wife was spotting me and happened to get me into a spot that as soon as both rear wheels found nothing but air for traction, a big rock popped up under the rear bumper.
When it comes to designing around stuff that will slide over rocks, it is all about the radius. If you take a bent steel slider with no tube on it, the radius is very small at the lower corner and rocks will dig in and stop progress. If you weld a 3/4 round section of 2" 1/4" wall tube to that same edge, it will slide like nobody's business and never hang you up.
There is also a reason I won't ever design aluminum for that application and that is because the radius is too small even for a larger diameter tube.
You have to pick material and use it where it works or put another way, don't use material that won't work for certain things.
If you consider that the radius of a belly skid is nearly infinite, the likelihood of hanging up on it is dramatically decreased.