It would seem advantageous from a fuel saving and time saving point of view to start the take off manoeuvre from when one is given clearance to enter the runway, thereby maintaining the kinetic energy of the aircraft.
I can see problems with the pilot's increased workload and the runway used as the engines spool up to takeoff power, but I'm sure I have experienced this as a passenger.
Do any airlines or airports encourage the practice to save fuel and/or time ?