A 20" full spare would take up considerible room in a trunk, but I have seen those goofy looking donuts that are the same diameter of the OE wheels, just a lot thinner,on some other vehicles. BMW has comparable wheel sizes on some of their later vehicles, and they seem to have no problem stuffing a full size wheel and tire way down in the trunk. Maybe the undercarriage is designed so it can't have a deep "wheel well" in the trunk due to exhaust pipe routing or gas tank being there?

I agree, a tire service kit won't help after some poor old chum drives it completely flat or even curbs it. They could at least put an inflatable spare in it like VW does on their Touareg.