In discussing this same point today, it was pointed out by someone much wiser and cleverererer than myself, that;
On the 320, if performing an APU->Pack Take-Off due performance restrictions and the APU developed a fire the APU bleed would be automatically closed, the engine bleeds would then open and in the middle of trying to fight a fire you might inadvertently find that obstacles in the take-off cone would look larger and closer than is health.
However on the 737 Classic which isn't blessed with such intelligence the problem wouldn't occur.
Therefore maybe SOPs for 320 APU->Pack Take-Off should be to also switch off both engine bleeds as a 'belt and braces' measure?