A few weeks back, when I wrote about the joys of the 15-year mortgage, I got the same reaction from a lot of people: "Why would you repay a loan when at these low rates, it's practically free money?"
I hear this about a lot of things. "Why would you buy a car for cash/save up to remodel the kitchen/have an emergency fund instead of a home-equity line of credit? Mathematically that's insane!"
I don't think it's quite as arithmetically unreasonable as my interlocutors suggest. Once you add risk into the equation, the calculations don't come out quite so neatly. By refinancing to a 15-year mortgage with a lower rate, we locked in 125 basis points a year, completely risk-free. And when the mortgage is completely paid off, we'll get another 3.25 percent, 100 percent guaranteed and risk-free. There are no risk-free investments that deliver that kind of return. You can make more money by adding risk — but you can lose more that way, too.