The reason given to me, as to why we don't use it on the B757 is that of PIO. The pitch up couple on the 757 is very noticeable and the pilot can soon get into problems if he does not know what the A/T is doing. Especially if it whacks on, or removes a load of thrust in the late stages of the approach. We tend to use it in the take off and climb, but then the thrust doesn't change much until Alt Cap.