Interesting thread.
We are directed by non normal procedure when we must use MCT. MCT use at other times is not addressed. By definition an engine can operated at MCT for an unlimited period of time. I use MCT only when necessary to increase the climb rate for turbulence or ATC request when a reduced climb speed is not appropriate or has already been selected. Selecting thrust up to MCT may also be necessary when a "SPEED SPEED SPEED" warning is present.
As Tom also said if I see a pilot selecting MCT in the mid to high 20's we talk about it at cruise.