Back in the day, AA also did "powerbacks". It's my understanding that it saved a good bit. A do know as a ramp controller, it sure did get the plane out of my way quicker, allowing me to give clearance to other departures sooner.
But times have changed. Fuel is a lot more expensive now. And the 727 no longer rules the skies, old DC-9s are gone and their MD-80/90 variants are dwindling. Mainline aircraft designs have moved away from the fuselage mounted engines back to having wing mounted engines. These engines sit lower to the ground, so are more prone to sucking in any debris on the ramp. For normal ops, it isn't too bad, but think about a powerback. The "reversers" are a series of vanes/buckets at the rear of the engine that deflect thrust out the rear to go forward instead. The forward thrust is actually blasting the ramp, kicking up debris and blowing it towards the front of the engines ... and of course the front of jet engines is a turbine, sucking in massive amounts of air. That's why, on landing rollout, you'll hear the (reverse) power drop to idle while the plane is still rolling at a fair speed ... while going fast, the front of the engines stay ahead of any dust/gravel/debris being kicked up ... as the planes slows, that benefit goes away.

Eastern Airlines was also a big powerback fan (I think they started it, but don't quote me on that).
They even powered back 757s (an airplane that was designed to sit high off the ground due to its modern powerful (read: fatter) engines).
I was at the gate for the AA 757 inaugural. Loaded up, she struggled to start rolling in reverse at lower power setting. By the time I got back to Operations, the bulletin was out ... AA 757s are pushback only.