Unfortunately this discussion has taken a regretful turn to misunderstanding and out of context assumptions.
Firstly you have interloped on an argument between an pilot and a ramp attendant. There is history here and both parties have issues.
Secondly regarding your particular flight, you fail to see the "Big Picture". Having an operating APU is a safety issue which you may not fully appreciate as you are only familiar with the short time you spent on this aircraft and are frustrated by the inconvenience it caused you. The Captain of this flight made a difficult call in that he is fully aware of both your personal frustration and the complications it causes the daily operations. Safety, operational integrity, customer service. Those are the priorities... in order of importance. Maybe your son can explain to you the hazards of operating in hot weather, on a busy schedule, on short segments, in confined quarters, with a full load of passengers, and only one flight attendant. Maybe he can't now, but after he has the experience and wisdom of the Captain you are questioning you would be more likely to understand and appreciate that particular decision on that particular day, on that particular aircraft, on that particular flight. "Sorry for any inconvenience, please speak to the agent in order to adjust your plans and request a passenger comment card to offer any suggestions to helping us serve you better."

GTFA