We found RVers in Alaska actually removing their stiff rubber rock guards because they were throwing rocks while driving a gravel section of the highway. We looked at their toads and a layer of large stones was laying at the base of their windshield. We could also see damage on the hood and front of the cars. Hard to believe that they all had them installed incorrectly.
We've also talked to owners while in campgrounds and found many had taken them off. We once considered installing one.
We didn't have a guard and we didn't get any damage through the whole summer in Alaska.
Once while driving the interstate during a rainstorm, for lack of something to do, I began noticing motorhomes going in the opposite direction. The ones with rock guards had a high arcing rush of water hitting their hoods. The ones without a guard had the arc hitting no higher than their tires. That convinced us!
We feel manufacturers install them for advertising their name. :)