BOAC,
That was a MUCH better response than mine. Well done.
I'm afraid this notion of passengers expecting compensation when murphy worms his way into aircraft systems really grinds at me... It is just the nature of the business. Often times it's hard enough to make the decision to keep an aircraft on the ground... we don't need the added threat of litigation to sway the minds of "production managers" who already too often push for wrong decisions.
In any case, yessir, any existing electrical or pneumatic difficulties, whether MEL'd or not, could mean they'd be counting on the APU. The expected temperature at destination could also effect the decision, as you know there'd be a planeload of lawyers waiting to sue for staining their shirts as they wait to depart.
I fear we've become a little bit too good at making air travel appear so simple that it's taken for granted as being no different than bus travel... sadly, it's even expected to cost less in some cases.