On the A320, why is it not recommended to use the HP ground unit when the APU Bleed is on?
The uses of the HP air of the pneumatic system are in the FCOM:


  
  air conditioning
  engine starting
  wing anti-icing (see here)
  water pressurization
  hydraulic reservoir pressurization
  


The HP air sources are engine bleed, APU, and HP ground connection. The Bleed Monitoring Computers (BMC) do not recognize if the ground cart is connected or not.

The BMC of the engines can detect overpressure and overheat and act accordingly (check FCOM for logic schematic). However, the APU logic does not check for overpressure and overheat (see below).

Having APU and ground as dual HP sources can lead to overheat and/or overpressure of the pneumatic system.

Even when starting an engine with external pneumatic power, engine bleeds should be selected off (no need letting the BCM catch an error for you).