i put about 3k miles a year myself. my Z only has like 14K miles and this is her 5th year of ownership.
for me, i dont have time to not drive my Z on a nice weekend so this is just a back up plan in case i encounter a blow out or watever as well. it's nice to know you have a spare set if anything happens.
if i only had one set, i would get one extra rear wheel with tire wrapped. 99% of the time i get flats on the rear tire from the front tire kicking up a nail or something. with asymetrical tires, you can use it on both left or right side.