One of the most common gripes about frequent flyer programs is that the miles you earn are hard to redeem when you need them. It’s not that airlines want to upset you; it’s just that they don’t want to give away seats they hope to sell. They want to let go only those seats that according to their calculations are going to fly empty anyway. An airline is a commercial enterprise that would rather make money than fly us for free no matter how much we think we deserve it. But if you are flexible, you can make the most of your miles.