Bi-weekly mortgage... scam-y

When you get a new mortgage, you'll invariably get one of the bi-weekly mortgage offers. I got one today.It's not really a scam since they do do what they tell you... but they're getting rich in the process doing what you can do for free.For the sake of ease, I'm going to use round numbers for the calculations.Let's say you have a mortgage and you're paying $2000 a month to. The offer would be along the lines of "every two weeks we'll take out $1000 from your checking account. You'll pay off your loan 7 years sooner!"The trick is that the third party is holding onto your cash and earning interest on it the whole time.The only reason you'd pay off your house any sooner is you're making an extra payment a year to the bank. You have 26 half payments compared to 12 full payments. At the end of the year they send in an extra payment -- with the money they have been earning interest on.You could do the same thing -- actually better by simply adding 1/12th of a payment each month: $2167 in the example above. The nice thing is that it'll immediately be applied to the principal!Even not counting the interest, you'd be around two fewer payments anyway.Attached is my long-form worksheet for it.  :-)Mortgage Comparison