[Jim Sims]

Depends on the size of the stone and trajectory; a couple of months ago my BMW's windshield suffered an almost normal impact from golf ball size rock. I doubt a thin layer of plastic film would have prevented the damage. The film likely helps with glancing hits from smaller rocks. In some jurisdictions, it is illegal to apply tinting or films to the windshield (windscreen) for various reasons - one being it impairs the vision of the driver looking out and the police looking in.