On the Mk 2 the problem was the engines would all start but there were no engine instruments until the AEO connected the power to the BUS. After a couple of engine fires, IIRC, the practice was stopped.
This was only a problem if there was no secondary AC supply at all. Normally the AAPP would be started by the AEO as soon as he boarded and AC became available via the Sync Bus. This was fine for the "mass rapid" but not if "Sim Start" was to be used. I also recall that when the Battery Master Switch was selected on this automatically provided a cartridge start for the AAPP if a start was selected within 20 seconds(?)*. If he forgot about the time delay and went straight for an AAPP start the result was quite exciting!
*IIRC this was related to the availability of cartridge start when the non-essentials had had been shed.
YS