A former Eastern Airlines pilot used to work in our group, and I asked him about power-backs. He said that at EAL, one of the labor contracts specified that the wingwalkers all had to be mechanics, and that meant you needed 2-3 mechanics available every time you had to push back a plane using the tug. To avoid the labor costs of having to have a large number of mechanics at every airport, they adopted the power-back procedure, which did not require having mechanics as wingwalkers. In EAL's analysis, the labor cost savings exceeded the fuel costs as well as the potential FOD risks.