Wow... You need to find a new dealer because the one who told you that either doesn't know what he's talking about, or is blatantly lying to you!
Many others on this forum have ditched the runflats for other tires. Personally, I would have done to same to get better grip and a quieter ride (my car has a spare too - not all GSs do). However, when faced with $150 for two used runflats vs. at least three to four times that much for four new tires, I decided to keep the runflats until all 4 needed replacing...
i think they took me for a ride...i was willing to get four regular tires. Im shocked that the spare is not fullsize, driving a GS with a flat is horrific