...except in emergency situations. Starting the APU was one of the first things they did during the “Miracle on the Hudson” incident IIRC. It’s conceivable that some in-air emergency happening prior to the loss of communication would have required starting it as a checklist item.

EDIT: I just wanted to add that aside from electrical power, it provides bleed air necessary to start the engines, run de-icing (also run A/C packs, operate toilets, etc.) —Hence its usefulness in an emergency situation.

I think we have agree to rule out that any of the problems affecting this situation resemble an emergency.

It can only be flagrant sabotage, or a blunder compounded by savage incompetance.

Either they fucked up so bad that they didn't just shoot themselves in the foot, but instead blew their whole leg off. Possible with hypoxia scenarios.

Or someone sealed the fate of that flight in cold blood. This version narrative includes the premise of suicide, including single-handed, lone wolf pilot suicide, although other nefarious possibilities fall under this umbrella.

The predominant fact of the outcome is that they flew so far off course, and without any distress signals, that their ultimate fate passed into history as a near total enigma which essentially requires strenuous effort or fantastic misfortune.

In some unusual scenarios the APU may be used during flight. If one of the engine's generators fails the APU might be used to help provide power, and below some altitude the APU in the 777 can provide additional bleed air.

In the 777 the left and right hydraulic systems are powered by the left and right engines, but the center hydraulic system has electric and pressurized air driven pumps that I believe can be run off the APU: http://www.flight-mechanic.com/wp-content/uploads/2017/11/12...

If MH370 lost engine power, maybe it was using the APU to keep the control surfaces active? Who knows. In that sort of situation the 777 would be a glider though.