This is such a trap. Even assuming the same interest rate you'd get for a 30 year, you'd end up paying ~double the interest in exchange for a monthly payment that's maybe a few hundred dollars less.

On a $500,000 house with 20% down (very unlikely for anyone taking this "deal") and identical 6% interest, you'd have a monthly payment of about $2,400 on the 30 year and about $2,100 on the 50 year. Over the lifetime of the loan, though, you'd pay about $450,000 in interest on the 30 year but over $850,000 in interest on the 50. With a lower down payment or less favorable terms--highly likely for someone going for a 50 year loan--you'd end up paying over a million dollars in interest on a $500,000 home.