Being from a family of mechanics, here is why you want to rotate your spare in.
If you do need to use your spare tire in the event of a flat tire or damage, you want your spare tire to have an even wear on it with the other tires. If you don't, you have the potential to doing damage to your vehicle. It would be like having 3 x 33inch tires on and 1 x 31 inch tire on. This could cause damage to your axles, ring and pinion and etc....I personally experienced this problem not by my fault. I had Goodyear put some brand new MTR's on my Jeep. After about 2,000 miles of driving cross country, I noticed a weird grinding noise coming from my Jeep. I got it in the garage and took of the diff cover and there was nothing but metal shavings. I contacted Jeep and brought my vehicle to the Dealership in Palmdale, CA. They were awesome; my warranty had just gone out. I had 71,700 miles on my jeep. That told me they would see what they could do, they contacted Chrysler and they agreed to fix it for free, my ring, pinion and front drive shaft were bad.
Well, about a day into repairs I get a call and they said that they were not going to pay for damage because I had an odd size tire on the passenger front. It was a 32 inch tire and the others were 33 inch and that is what caused the damage. I explained I just had the tire put on. So, I called the Goodyear that installed my tires and explained the situation, they called the Chrysler dealership and long and behold Goodyear agreed to pay for the damages. They admitted that there were a bad lot of tires that were improperly labeled on the white labels that go across the tire.
So, back to my point, rotate your spar in to allow for even wear on all your tires in case you ever have to use your spare. I would rather pay 300-450 for a tire than a few thousand for repairs caused by running a spare tire that was not broken in.
Just my two cents, others may disagree.
Sincerely,
Mark