They aren't a bad idea as long as you understand them.
If the market is stagnant, then an IO mortgage which you choose not to overpay can be effectively a cheap rental deal. The house is owned by the bank and your "rent" is the interest payment which is much less than it would be as a normal rental.
If the market grows steadily over the course of an IO mortgage then it is possible for a couple to pay the interest on for their large family home in an expensive area for 25 years then sell, pay back the original capital and have enough equity left over to buy a cheap smaller place outright.
If you want to repay the mortgage over the course of 25 years but have a variable income (eg creative professionals, consultants etc) depending on many short term contracts with fallow periods in between, then IO is ideal as you can live frugally when income is low and make lump sum payments into the mortgage when you can.
If you are a canny investor and can identify areas of the financial markets where return on investment will be higher than your mortgage interest rate then instead of paying off the capital of the mortgage directly, you can invest the same capital-repayment money into other parts of the financial market and hopefully after 25 years you will be better off than if you used the same money to pay off capital. This can go wrong if financial markets don't produce a good return.
IO mortgages work well for a lot of people.
They go horribly wrong if taken out by idiots who don't understand what their obligations and risks are. This is true of many financial products.