Why does the AAMVA manual say you should never use the trailer hand valve to hold a trailer in place for parking?

AAMVA CDL Manual, Section 6.2.1 "Trailer Hand Valve," p. 6-4

A trailer held only by the hand valve can roll away hours later once the air leaks down, with the engine off and no one around to notice. The manual's rule, use the parking brake and wheel chocks if there are no spring brakes, closes that gap entirely.