Originally Posted by jmmushkin

The math is right, but only if there are no visas inside to worry about replacing!

The visas from many countries last only as long as the current passport is valid (or less), so getting a new passport can be extremely expensive and inconvenient if one has already gone through the bureaucratic process of getting a visa from, say, Saudi Arabia or India or Brazil. Even a simple one like Chile's, while not requiring any annoying mail-in shenanigans in advance (it's done right at the airport upon arrival), is only good for one passport (and it's over a hundred bucks!).