What in the world is wrong with a big interest rate on a miles-earning card?

You should never carry a balance from month to month on a miles-earning card, because the interest (even if not "big")) more than wipes out the value of the miles you earn.

If you are conerned about interest rates because you're going to carry a balance, you need to get the card with lowest interest rate regardless of whether it gives you any miels or not!