Because some airport people seemed really stunned that anybody would fly anywhere just for miles from time to time, I have to believe it's so small that they just don't see it all that often.

I would venture single digits, if not a fraction of a percent.

There is a difference between a mileage run and a family vacation to take advantage of a sale, if you count in the business trips and vacations it would be higher, though even most of those people are not doing it for the miles.