Mike, on the contrary, rather than buying two identical pairs of shoes and wearing them alternately, it's not a bad idea to have two DIFFERENT types on the go at once. Even though you will need the same broad category of shoe each time (cushioning, support etc), the design differences between models and manufacturers are big enough to shift some of the weightbearing stress from one muscle group to another. For example, I am currently alternating a pair of old-style Nike Air Althea (the new model is rubbish, but don't let me get started on that) which have 400 miles on them with a pair of Mizuno Wave Riders which have about 60 miles on them, and walking in expired Asics 2060s - all suitable for lightweight mild overpronators, all very comfortable (I am as fussy as can be about running shoes, and won't buy something that feels as if it needs "breaking in"), but all very different to run in.
My approach is to buy a new pair when the next newest pair has about 250-300 miles on board, try them out on the treadmill a few times, then let the oldest pair become walking shoes.
It could all be pure pseudoscience, but it's an excuse to buy lots of trainers!
