For people who are intending to eventually own the home, the stategy is that in good economic times you can pay down the principal at an accelerated rate, short

For people who are intending to eventually own the home, the stategy is that in good economic times you can pay down the principal at an accelerated rate, shortening the term of the loan, and in bad times you can fall back to interest only payments. It's a safety margin. Japan in the 1980s had 200 year loans with the same rationale.