The 100mm rule would be in effect, to prevent you from bottoming out over every dip and bump in the road. Bottoming out can be dangerous, as in extreme circumstances it could cause one or more tyres to lose contact with the road, hense the potential to lose control. 100mm would be the figure decided on, that would prevent you from bottoming out on anything other than the most extreme conditions.
By the way, I have seen plenty of stock cars scrape on some driveways.