Historically reservists ID cards are only usable when accompanied by orders stating "active duty status". I am sure I didnt use the formal words, but jest-during those periods of active duty - as student - those summer training weeks, or as reservists doing their 1 weekend a month - the member can use all privileges.
I do not know if its different now -my son is NROTC cadet - too has the reservist ID card -I understood from his unit llast year, that it would only be effective while on active duty - someone else will probably correct this information for you - but that is my understanding- larrys mom