With the plastic bicycle or car type cap, agreed - they do not act as a secondary seal, but aircrat ones do! As said earlier, they are designed for the function of keeping air in the tyre in the event of primary seal failure. (Metal + torque setting... usually 6lb/in, ie. calibrated-finger tight).
As a ramp tramp (semi-retired) I would think twice about grounding an aircraft with a missing valve cap... just imagine trying to explain that one to the CEO.. but for the jockey to put in an ADD, no problem. It'll get done ASAP.
In Hitchers defence, flight safety is everyone's problem, so full credit for spotting the faults. As a security guard, his responsibility will cover the ramp and the aircraft on it, so I can see why he is close to the aircraft. He did not say he interfered with the aeroplane.
Can we just all get along please?