Sal,
I don't know about the cost, but IMHO it is cheap insurance. I've seen way too many nice cars covered with rock chips.
The only thing I wonder is the clear film must change the light as it enters through, if not even by a little amount. Is it possible that this film magnifies heat and/or light and may speed up or change the UV fade in the clear coat?
Along the lines of decals on a squad car (or any car for that matter) when the decal is removed you can see how bright the paint used to be under it and compare it to the natural UV fade around it.
Just something us crazy detailing nuts think about...