I think that ground crews would prefer powerbacks all the time, but the airline doesn't for a couple of reasons:
1.) Fuel, you have to start both engines early.
Most of the time, after push back, and taxi they will only have one engine running, and start the second, on the way to the runway.
2.) Noise regulations.
They are very noisy, they have to spool up pretty high to be able to move.
3.) Some airport prohibit them because of tight ramp areas.
If there are aircraft/gates directly behind they will get jet blasted. Remember before the thrust reverser opens, he moves forward about a foot, to start a momentum, then engages the reverser. Plus it is hard (or impossible) to stear while in reverse. So they have to have space behind them.
N 7 5 7 T W
p.s. Hope my facts are correct, my Dad told me about it, when he worked for OZ/TW.