Here's my opinion based on decades of working in the automotive industry and being an avid off-road enthusiast.
Average tire life is five years based on the average car. The industry says the average person drives around 14k miles a year.
There's your baseline for anything you read on the subject. Notice I didn't mention specialty off-road tires and modded vehicles?
I live in So Cal where heat, UV and smog will lessen tire life a little. My spare lives on the back of my vehicle exposed to the elements. If I do a 4 tire rotate, the spare just rots. 10 years later, I have a new tire I cant trust sitting on my swing out. People like 4 tire rotations because when the tires wear out, they only buy 4.
I do 5 tire rotations. If I have 20% tread left on my tires and get a flat, I put similar sized spare on not a full tread tire. Does it make a difference having two different sized tires (1 new, 3 old)? It can. When I used to run rear LSD diffs you bet your butt it did. Some newer abs/traction control systems will flip out also. Based on a 2500 mile rotate, your also suppose to balance every 5k. You should see how that extends tire and suspension life.
Opinions will differ. What you do is going to be based on how your vehicle is modded, what the vehicle is and your preference.
