If I remember correctly from an article I read some time ago, the Laborghini Diablo was not supplied with a spare tyre. It was however supplied with a can of the quick-fix.
They also made a good argument in that the spare tyer was a leftover from the days when tyers were basically a big balloon. Modern tyers do not "pop" as they did in days gone by - even with a nail stuck in them. Its very very rare to get a flat these days without lots of advanced warning... enough to be able to drive the ofending tyre to a repair shop.
Another thought. Maybe if one was to fill a tyre's-worth of space with other spare parts such as radiator hoses, coils, emergency fuel, jumper-leads, ignition leads etc etc you would probably find yourself getting stranded far less often than if it were taken up by a solitary spare tyre.