I was thinking about this recently. A friend and I were flying WN, last year I believe, and he got called to the gate and given free bonus miles. I was a bit surprised as I have much more flights on WN than he does, but he tends to fly WN on one leg and return on a different carrier. I was wondering if that was why WN gave him bonus miles to try to get more business from him.

Ever since that experience, I've started more mixed routes where I fly WN one way, and a different carrier on the return as well. Have not been called yet to the gate for the bonus miles though.