Could it be that the APU has auto shutdown for certain conditions and certainly when in the air we don't want the APU cooking or overspeeding with no fault indications, this would be a hull loss for sure. The Engines are both visible, have two fire bottles available, have numerous other indications of malfunctions and seizures. The dual loop system will still operate with a fault indicated whereas the APU system will not. And of course, you can identify which loop is inop on the engines ,Just a thought for the melting pot.