Once (and only once) about two years ago I was checking in on a coach ticket on a 3-class trans-continental (UA). The RCC agent told me that I had been given a "complimentary 2-class upgrade." I was only premier at the time, and the plane was only about half full, so I never could figure out how or why. At the time I assumed it was thanks to my having just surpassed 25,000 miles for that year, but in retrospect, I can't see why they'd appreciate that so much...