Many New Cars Have No Spare Tires

...my wife's 05 Grand Caravan has a spare underneath that has never been used and the rim has rusted and the tire has rotted. Do I replace it just to have a spare?

Nope.
My '12 Caravan has the spare under the chassis too. It has a big lug nut under the front console that you turn with the lug wrench and it lowers the tire down so you have to get under the car to drag it out. Question: If the van has a flat, wont it already be sitting pretty low so as not to be able to get the spare out? Our 98 Suburban has the spare in the back behind the seat. You mean you could not find room UNDER the truck for a spare? C'mon....