There is a credibility risk to not being held accountable to a single target, but there is also a risk to regularly missing that target, even if it is due to factors outside the control of monetary policy. But, that does not mean that some clear and, most importantly, achievable, goals should not be articulated.
Take two of the broadest of institutions, a market economy and a centrally planned economy. In a market economy, the market price signals any excess demand or supply, and creates incentives for producers. When the market price is higher than the equilibrium price, a producer has an incentive to increase his or her production and increase sales.