It's your extra money spent on a new cassette and or chain rings if you leave your chains on until they actually skip.
And although unlikely, if you ever have a chain breakage, if it's been on the bike for donkey's years, you will have to buy a new cassette also cuz the new chain will skip on your worn cassette-which will cost x amount depending on the cassette, and from a tourers perspective, might not be available when the broken chain scenario happens.
You pays yous money and take yous chances.
I stopped tracking mileage when my last speedometer/odometer broke down after a heavy downpoor I rode in. After several years. That was a 3x8 drivetrain. Experimented before with a 3x6 drivetrain. I also fix bikes for friends and acquaintances and work part time in a LBS. My calculations are mostly based on cheaper drivetrains For those: if you want minimal investment of both time and money, it is cheaper to use one chain until it starts skipping. Yes it does require a new cassette, sometimes even a new set of chainrings - but it is still cheaper than changing chains regularly. The price paid is less crisp shifting and a higher risk of chain breaking. So it is out of the question for riding with lots of climbs. But for flat land riding - it is both quite safe, convenient and it is cheaper. Not sure about highly priced aluminium front chainrings (and cranksets). Haven't done any math or testing on those.
I love riding in the mountains and prefer crisp shifting to saving a few $, so I change my chains regularly when worn. But I understand houeyhoolihan's logic - it can make sense for some people, like I explained.