Forget rising or falling markets with regard to IO mortgages.
They are a bad idea full stop, regardless of what the market is doing.
You are taking money off someone, which you are not paying back, and are paying for the privilege of having it. I can think of few instances where doing this is a good idea, no matter what the market is doing.
You're average borrower has no idea if prices will be up or down two years from now, so it should not even be a consideration, or reasoning for taking one out, or used in the logic for justifying it or not.
And what happens when your payments reset after your allotted time period and you're finances are not in as good shape as you hoped they would be (Again, people always assume they'll be on more that the are in reality.)
There are very sound principals to borrowing money which have been established for a long time, and people should heed them.
You don't get something for nothing, and people should exercise some more prudency when it comes to matching their aspirations with their reality.
Granted you'll no doubt hear of cases were people fluked market timing, just as you'll hear stories of people who got hurt with IO mortgages. Are you willing to bet your financial future on a 50/50 shot?
Most of us would agree that buying a house is a good long term financial plan
Putting everything you have into one asset on an interest only basis is not a financial plan. It's a high risk undiversified bet, and people should look at the whole picture before running down the garden path.
Again, if you can't afford to pay it back properly, you can't afford it full stop