being a member of staff but useless as a worker
A set phrase criticizing someone who holds a position but contributes nothing; literally 'merely being counted among the staff'. Used in workplace or organizational contexts.
He is just a name on the roster and contributes nothing.
Such an employee is merely a seat-filler.
員に備わる means to be counted among the personnel or members; のみ restricts this to nominal membership. The expression thus describes someone who fills a place on the staff without performing a useful function.