I absolutely get what you're saying and every time I load up the cargo space in my car for a trip, I think about what a pain it would be to have a flat and have to unload it.....however, I live in NY, no way I would buy a car with the spare tire location being underneath the car. I would not look kindly to laying on top of salt encrusted semi-melted slush/mud in 30 degree weather at night along side an interstate while learning new swear works trying to get my rusted spare out.
I was just looking at a Hyundai a friend has that in order to get the spare out from underneath the car, you first had to lift the cargo floor cover and then access a large slotted plastic screw to lower the tire. Well, in order to lift the cargo floor cover, you have to remove most of the luggage and once done with that, if the rust is preventing the spare from lowering, you will end up fighting with it between going into the cargo area trying to turn and probably strip the plastic screw because the rusted tire/wheel mechanism won't lower, then back under the car to see what's going on and try to wiggle the tire down and then back up into the cargo area to mess with the screw and back and forth, back and forth...fuggetaboutit. Give me a full size spare located inside the car.
Now, the issue with the actual tires being bigger than the spare located inside the car is something I have not realized but you bet that will now be on my list of things to verify when looking at cars....ugh!