cyrus,
the main reason why you would NOT use APU for aircon during takeoff is that the computed thrust for takeoff, once set, is maintained.
if APU shuts down during the takeoff roll, the aircon bleed source automatically reverts to engine bleed.
to satisfy this unscheduled bleed demand while maintaining the original computed takeoff thrust , the FADEC increases the fuel flow, thus increasing the possibility of very high EGT during the takeof rol.
this is in addition to the possibility of engine exhaust ingestion into the aircon system thru the APUl
