I did say the only thing you need is air, so no APU=not dead in the water.
I've left it on once and noticed it at 1000 MSL and switched it off... I've heard leaving the APU on results in everything from it just flaming out to blowing up the tires to fire bells to compressor stalls (when switching onto #2 bleed in flight) etc. etc. I've never let it get that far and the most common one I hear is a flamout.
I'm not sure how much they taught you at EA, but I've had to climb under the LE flap more than once to override a premature VTO shutoff.