Saving time was always the reason I heard for adopting the powerback practice, the theory being you can powerback and begin the taxi much more quickly than using a tug to pushback, having to disconnect the tug, waiting for the tug to back away, etc. I have also been told by pilots and mechanics (at airlines that use a tug for pushback) that powerbacking is hard on the engines in the best of conditions and, as mentioned above, there is always the danger of FOD (foreign object damage) from ramp debris.