A recent post (via Mark Thoma) at the Federal Reserve Bank of New York presents this argument in detail.
"The language used in the press and elsewhere is often imprecise on this point and a source of potential confusion. Reserve balances that are in excess of requirements are frequently referred to as “idle” cash that banks choose to keep “parked” at the Fed. These comments are sensible at the level of an individual bank, which can clearly choose how much money to keep in its reserve account based on available lending opportunities and other factors. However, the logic above demonstrates that the total quantity of reserve balances doesn’t depend on these individual decisions"The full post is required reading for those who want to understand how the level of excess reserves is determined.