money pays down your principal, right? Usually a good thing.
Also why would an ARM be good now when rates are near their lows? Seems like a perfect time to fix it.
Finally why in the world would you put an investment property on an interest only ARM? That seems disastrous. What happens if we see an increase in rates in 7 years? A fall in housing prices and now you have to sell because your ARM is up?
I have seen people get interest only ARMs for good reasons, I just don't think you provided any.