Valve caps aren't fitted when leak testing tyres over the recommended period of time after wheel build-ups so I can't see how that makes them a primary seal...
Notwithstanding my opinion, here's what my Michelin Aircraft Tyre Care & Service Manual has to say on the matter:
Each valve should have a valve cap on it to prevent dirt, oil, moisture and other contaminants from getting inside and damaging the core. (Page 47)
Valve caps, finger tightened, should be used to prevent dirt from entering and holding open the valve stem. (Page 49)
(This is probably the reference source spannerhead made earlier but the link wouldn't open.)