Rotating two or more pairs of running shoes can produce the following benefits:

(1)   reduced risk of injury.

(2)   extended lifespan of shoes.

It takes up to 48 hours after just a few miles of running for the midsoles of running shoes to fully recover their shock absorption properties. Allowing them time to recover at least that long reduces one's risk of injury and extends the life span of the shoes. In his book, “The Competitive Runner’s Handbook”, Bob Glover says, “Studies show that by alternating two pairs of shoes they’ll last longer than three pairs used consecutively.” He also says, “Rotated shoes retain 80% of their cushioning after sixty runs of an average of 5 miles (300 total miles) compared to only 60% for those not rotated.”

As long as one doesn't run on consecutive days, it isn't really "necessary" to rotate multiple pairs. However, it's still a good idea in order to stagger the "age" of them. That enables "retiring" one pair while others in the rotation are still "young".

I like to rotate three pair. When it's time to retire the oldest pair, I have two other pair in the cycle with 1/3 and 2/3 of their lifespan remaining. I usually use the "youngest" pair for racing and speedwork. For a marathon, I introduce a new pair into the cycle a couple of weeks before race day, get 25-50 miles on them, and then wear them in the race.

People who are biomechanically neutral can run in different types of shoes (motion control, stability, cushioned, light weight trainers, racing flats), as well as different models, for different purposes....daily running, long runs, speedwork, racing, etc. Although I run in one model of shoe today, I used to have up to three different models in my rotation, but all were of the same type (stability), which I need to control my moderate overpronation. I once introduced a pair of light weight trainers into my regimen in order to have lighter shoes for races. Within just a few months, I developed iliotibial band syndrome (ITBS) at the hip because the light weight trainers didn’t adequately control my overpronation. Although I wore them only 1/3 of my mileage, that was sufficient to result in the injury.

How do you know when it is time to “retire” a pair of running shoes? Some people mark each pair….1/2/3, A/B/C, purchase date, date of induction into the rotation, etc. Some people log/track mileage on each pair. Or you can cut a notch on the edge of the sole after each run. ;)