If monetary policy bases its decisions on just one single equation or set of equations, it effectively assumes to have perfect knowledge of the key structural aspects of an ever-changing economy. In a complex and changing economic environment, a mechanical use of fully specified decision rules is too restrictive to ensure an optimal, robust policy.
Simple rules limit the amount of information used and underrate the need for judgement in the use and interpretation of any economic model. They are no substitute for an ongoing articulated analysis of all the economic and monetary forces at work in the economy and for the need to check all policy proposals for their robustness across a variety of frameworks.