X2 - What she said.
I was once doing the whole CDL brake check routine on a brand-new Freightliner, while being ribbed by my partner who was also a volunteer fireman. He was kidding me that if I did all the checks like that on a pumper, it would never get off the floor to a fire. I told him if I were a firefighter, I would do it for the weekly truck check, not an alarm.
When I got to the part about building up pressure and then shutting down with my foot on the treadle and listening for leaks, sure enough there was a slight hiss up front. The master valve got replaced by Freightliner under warranty, and I never got kidded again.
EDIT: There are a whole lot of perfectly usable vehicles with air brakes that do not keep up pressure when parked long-term. But I would want to know WHERE it was losing air, if only for my piece of mind. I also would want to do the refill time test to know that the compressor could fill the tanks faster than they leaked.