Company SOPs
When I worked for a Big Airline some years ago the management became concerned about "incidents" occuring when Airbus crews practiced/used manual thrust on approaches.
If I remember correctly (getting old memory fades etc), they eliminated the problem by banning their crews from practicing the use of manual thrust while line flying.
Needless to say being managers and cynics, they were quite happy to keep the MEL unchanged so if the autothrust bust one was expected drive in and out of LHR and other busy places with manual thrust....
Bigpants