Hi Challanger05,
The instructor who gave you the information is in fact correct and I was wrong. The procedure to which I referred was for takeoff with APU door open. I always assumed (wrongly) that it was to allow you to make corrections should you despatch with one engine generator inoperative. However on closer inspection it does say with APU shut down and in fact the MEL does not allow you to despatch with one generator U/S.
You do get the APU fault message on takeoff for the reasons specified but as long as you are aware it will happen it does not cause any significant problems. However, due to the inability to despatch with a U/S generator I cannot think of a situation where you would need to have the APU running on takeoff.
Cheers,
TSP