I assume he's talking about the act of taking a Mileage Run, which FT'ers define as a trip for the purposes of earning miles that yields more value to the traveler than its cost in terms of money and time - and therefore less revenue for the airline on the subsequent award trips.

Hard to say whether that's true or not. In a world without miles, the Mileage Runner probably takes neither trip (neither the MR or the award trip that it spawns). So the question becomes whether the traveler found an MR that was so good that the airline actually lost money by transporting the traveler and his luggage all over God's creation. And that I don't know...

(Obviously I'm oversimplifying things a bit by saying that someone takes an MR and then uses those miles to redeem 1 award trip.)