This is my take too. A $10,000 purchase on 0% interest for a year earns you ~$400 in interesting from a money market fund or whatever. I'm in a high tax bracket so $160 of that goes to the government. So even for a large purchase I net $240 in a year. Hooray.

Decrease the total cost of things purchased in this manner or increase the number of different things to track (or both) and suddenly I'm paying quite a bit more in additional mental energy than I benefit in cash.

The only place where this makes sense (IMO) is when you buy a new car and can get 0% and no payments for more than a year