Something else to keep in mind - the full size spare tire present in most North American cars is there because it forms part of the impact protection system, to meet the safety specs that deal with impact absorbtion, fuel tanks not exploding like Pinto's when you get rear ended, etc.
The fuel tank issue might not be a big concern for us, given that we use diesel fuel, but our Golf's and Jetta's don't have a whole lot of bodywork hanging out back behind the rear seat passengers. I don't think I would want to further reduce impact protection by removing the spare.
There was a study done not so long ago inquiring into why so many police officers were injured or killed when their big Ford Crown Victoria's were rear-ended. Seems the common casual factor was that they had all removed the spare tire from the trunk well, to allow them to store other stuff in there.
Michael