A constant rate of money supply will not always be optimal, but it will keep you out of a lot of serious trouble you could otherwise get into. The Federal Reserve and virtually all other central banks operate in a policy regime in which we set some short-term interest rate -- in our case, the federal funds rate.
A demand shock requires a shorter horizon for the policy response, that is policy should be less patient, whereas a supply shock may call for a longer horizon, and hence more patience. Wouldn’t it make sense to allow more downward scope in the inflation aim? The aim must be crystal clear, especially when inflation is low.