Dream Land was right to insist, the valve cap is there to prevent leaks and dirt ingress.
From the AMM wheel inspection A320
4. Procedure
NOTE: The data given is for bias and radial tires and for all Airline Operators.
Subtask 32-41-00-210-070
A. Visual Inspection of the Wheels
(1) On each main gear wheel, examine the drive shroud of the tachometer.
(2) Make sure that there are no cracks on the wheel rims.
(3) Make sure that there are no leaks on the valves.
(4) Make sure that the valve thread is not damaged. Fully tighten the valve cap which seals the valve from dirt and leaks
I read from his original post that this was deferred as an ADD and Dream Land asked the engineers at his main base to sort it when he got there. Exactly the right thing to do in my opinion.