Mileage may not be such a moot point when it comes to spark plugs, in my opinion. The life of the plug is affected probably more than by anything else by the number of (zillions) sparks it has given over time. It is not exposed to any kind of weather effects like hoses and tires.
I changed plugs in my car several times after many tens of thousands of miles. The electrodes were a little more rounded compared to the brand new plugs, but you had to look really up close. I saw really no difference how the car ran after I changed them (no mileage difference, no difference in the smoothness or anything). I often suspected they would have probably lasted another 30k or 50k miles -- assuming the proper gap adjustment.
Maybe those of us who have changed plugs after a long interval can share their opinion.
Don't get me wrong, I'd hate to skimp at a wrong place - I know we are talking a few bucks and a 20 minute job, but at the same time I do not replace say the brake lever on my handlebar every other year just because it is easy to do and it is cheap. I hope you get my point.
'07 Vulcan 900 Custom, Candy Fire Red
'05 Suzuki DR-Z 400s