"Rubber" is like "wood", it's all the same right?
Like lignum vitae, which is harder than some metal alloys. While balsa can be crushed in one hand.
Folks who live (or drive) alongside RR tracks have a special problem because of iron particles in the air, thrown off when steel wheels roll on steel rails. Folks who live under airport flight patterns get lots of soot, from unburned jet fuel (kerosene) when the aircraft are landing and taking off. All kinds of circumstances are invisible to the average driver--but they exist.
In some markets no one gives a damn about a patched hole or a patched fender. In the *larger* markets where they have a bigger choice of used cars? More likely to compound out some scratches, than to try getting a roof repaired--which involves a major paint job when the 'roof' isn't just a small distinct body panel. That one small plug in a roof mount's hole will probably reduce the value just as much as having a fender torn off the car, in the large markets. People would rather just buy another car, and not worry about leaks. Which no used car business is going to give you a guarantee on.
"...a non-porous conduit for moisture..." Eh, funny phrase. At some level either it IS porous or it ISN'T. If water can get through it at any level, it is porous. Just like Gore-Tex, which is both waterproof and porous. Get into a car with heated seats on a rainy day, wearing a wet Gore-Tex jacket, and you'll quickly find out how porous it can be!