One of my friends is an AA 73 captain and he said there's a lot of cultural opposition to taxi on one, and also they have pretty strict taxi thrust limitations that are hard to work with for the 737NG. At Spirit we used to have a 40% limitation, which was removed around the time I became a captain. Definitely made it easier as sometimes 40% just didn't cut it.

But yea, I've been on AA where we had a 40 minute taxi out and they ran both engines the whole time. That's fuel you could have in your tanks and could make the difference between diverting or not later in the flight.