[crackered]

Further, because of the amortization schedule, applying that extra principal payment only shaves off the last month of the amortization schedule, which is the smallest fraction of interest of all payments.

One would still be better off investing that money in something until that last month, then apply the payment to save the very small amount of interest.

This assumes the mortgage is like most (all?) mortgages out there that follow an amortization schedule -- which are unlike credit cards or student loans, where early payments have a big benefit.