Powerbacks can be used for several reasons. Most of them have been mentioned here. They are hard on the engines, regardless of the engine type. One other problem is that the pilot is essentially going back "blind" and completely reliant on the person marshalling the plane in terms of when to turn, stop etc. During a pushback, the reaction time is faster if you need to respond to a problem. Powerbacks are also very noisy procedures and, as mentioned, can only be done on T-tails due to FOD. As an ex "ramper" for an airline, the only time they saved time is when the pushback tug was acting up or unavailable.