Nw does indeed powerback DC-9's and until they retired them 727's also. But years ago Eastern used to power back 757's. But NW does not powerbck anything other than 727's and DC-9 because like you said it increases the incidence of FOD (foreign object debris) into the engines. Although powerback does save the trouble of having additonal personnel. I know that powerbacks are hard on engines and I suspect that practice lowers the overall life of those engines.