There is more to the button than getting full thrust. It changes the mode for the flight director, subject to the constraints I mentioned in an earlier post. There is something to be said for just flying it like any other aircraft. You know, cows get big, cows get small, push the go faster levers etc.
Is the imperative not to get the aircraft clean away from the ground in some unpleasant period of uncertainty with the two pence/half pence feeling in the seat of the pants?