Hi again,
Thanks for all your replies, but as a matter of trim set to avoid excessive force on yoke by pilots to counter pitching moment (this is what I have heard often) , this does not seem to be logic for me because ( the case of commercial aircrafts), the effort that moves the moving surface (elevator) is furnished by hydraulic servos (or electric motors), not directly by the pilot muscles, and thus these hydraulic servos and motors are designed to move the elevator at all conditions of flight (included take off at all allowed cg's).
Just think of FBW (fly by wire) aircrafts where the pilot's arm just feels an artificial feedback force proportional to the real force applied to the elevator.
Then, the purpose of trim set at take-off is still not clear for me.