I used to fly about 100k miles per year. All were domestic US flights. At the time, there was usually little difference in fares between the majors. I was travelling between my home (BOS) to either ATL, DFW, or ORD. Flying to DFW mean that I was going on AA, period, end of story. They had direct flights and lots of them. ATL meant Delta. Chicago meant either AA or United. Since I had Platinum status on AA and the fare was the same, I chose AA when going to ORD. Did I ever go out of my way? Heck no, I wanted a direct flight (no stop-overs or changes, please). The sooner I got to my destination, the sooner I could start billing hours and the sooner I could go home.
Regarding who was the "decision maker" regarding the airline, I was, even though I was just an entry level consultant in a large consulting company. If I abused the system, our travel agent would have told my boss (who had to sign my expense reports anyways) and he would have taken me out behind the woodshed. It is not the job of the airline to act as disciplinarian for our company. That's not your job, that's our job, so don't try to do it for us -- you'll only muck it up.
Regarding giving the mileage to some purchasing hack, I was the poor slob traveling 50 weeks a year, waking up bleary on Saturday AM from a midnight arrival only to start packing on Sunday PM. I was the one who blew out my feet waiting in line at airports, not him. And if I have the choice to take AA or United to ORD, if the fares are close and one of them gives me perks (upgrades, miles, etc.) and the other doesn't, you can bet which one I'm taking.