Let me get this right. You think you should perhaps leave the APU on up to 10,000' every single flight, burning say 50 kgs of fuel, every single flight, just in case of the once in a career (approximately) possibility of an engine failure and loss of a generator, in which case the remaining generator will have load shedding take place to reduce load, and besides which you will still be able to light up the APU if needed to provide you with an additional electrical source? Am I seeing it right? Why?
How frequently are you expecting engine or generator failures down there Bro?
I flew 732s for 6 years, 734s for 2 years, and now 737NGs for 18months, interspersed with various model 747s. The last engine failure shutdown I had was roundabout 25 years ago, on a 747-200. Never a peep out of any model 737 engine. I have to keep the APU going on ETOPs flights. I have had a generator failure recently on a 737NG at 41,000'. The APU, much to our surprise, started and handled with no problem. Loss of a generator is not a serious problem. It is really no great help having the APU running all the time, and is a positive fuel waster.
Look on it as an insurance policy. You are paying the premiums of all that APU fuel every flight. What is the benefit? A once in a blue moon benefit that is not really all that useful? Worth it? Nah.