Traditionally monetary policy is only concerned about price stability, particularly in the inflation targeting regime, even if central banking has since the XIX century cared for financial stability in the form of lender-oflast-resort for banks in times of liquidity distress.
This asymmetric policy of only “mopping-up” with liquidity in financial market downturns but ignoring the risks of the upside booms creates moral hazard and provides a significant “liquidity subsidy” to the financial sector. The arguments against doing otherwise have been centred in the difficulty of “identifying bubbles” or in the danger of “pricking bubbles”.