I've answered this before, but I'll bring it up again because no one mentioned it here yet.

I use my accumulated reward miles for the annual New Year's Eve trip I always take....it net's me about 15,500 "status" miles when flown as a "paid" round trip.....but in late December I'm well past my status qualification for the year, and I don't want to spend real money on mileage accumulation when it gets split between two years and wasted as a result. So I empty out my account for 1-2 flights.