Attempting an APU start can drain the battery rapidly. If the start is unsuccessful you'll be left with a dead battery and electrical services dependent on the RAT. As the RAT will stall below about 140kts you could find yourself left without instruments or controls at the most critical phase of the flight. I'd probably consider one start attempt if at high altitude and time permitted, but with a max altitude of 3000 feet on the flight in question I think I'd have higher priorities than starting the APU if the aircraft was still flyable.