Don't you think Airbus would have recommended starting the APU if they thought it was desirable? You do not need to turn the A350 into an A330.
Presumably for the same reason they don't recommend it on the A320 or A330 series, yet nearly everyone does it anyway? I'm guessing that if it becomes a recommended line of the ECAM (eg "APU (IF AVAIL)... START"), it might have presented problems when writing the MMEL.
Don't have the A350 FCOM or MEL - can you dispatch with APU and 2 VFGs inop?