The cap is a secondary seal. The primary seal is part of the valve core. The cap's functions are:
1: Prevent debris from entring the valve stem and depressing the core
2: To act as a secondary pressure seal in the case of core failure.
On an A320, a single tyre, let down under reletively controled conditions (core failure, so no explosive decompression), would not cause the aircraft to be uncontrolable on the ground. So worst case scenario is that you have one flat tyre. Its not in the MEL, so engineers must use COMMON SENSE / ENGINEERING KNOWLEDGE to dictate whether the aircraft should fly or not. Thats why you get paid big bucks!
