Why would you bother with a 1 year mortgage?
If you are at the end of the mortgage term and have less than two years left to go, it might be the best way.
If you have a number of years left and want to maximize low interest rates (ie, 6 years left at current rates, do a 1 year mortgage at current low rates, then lock in for 5 years at presumably low rates, if you think they are going to go up a lot in 2 or 3 years).
If you expect the days of prime minus .75-.9 to be back in a year, it might be worth it to get a one year variable mortgage now which is only around prime or prime - .1.
If you are thinking of moving and want to keep terms to a minimum so that if you do decide to move you won't be paying a massive interest penalty.
I'm sure there must be others, those are just the only three reasons I've ever considered/recommended one.