I have to disagree with your disagreement.
As mentioned by Shadowfax, the right type of storm will do two things:
Punch a hole right through the fabric;
Lift up, and carry away the whole canopy.
Some of those tent designs only protect from hail coming straight down, and won't protect hail coming in from the side, circumventing the canopy.
If they stay put, and the hail is small, they might work fine.
Larger hail, or a severe storm, not working so well.
Plus, again, there's the logistics nightmare of putting up a huge car covering tent in the middle of Denver, or any other city. Won't really work for most people.
As for the poster further down that recommends some sort of bubble wrap like cushion, that really won't work well either, as the hail would hit the bubble, pop it, then the next chunk of hail that hit the same spot would then cause the damage.