Simply printing money only expands the monetary supply, it can never contract it. Government bonds on the other hand have the nice property that they can be use

Simply printing money only expands the monetary supply, it can never contract it. Government bonds on the other hand have the nice property that they can be used to contract the money supply (by selling bonds) or expand it (by buying back bonds). Also, modern financial theory is predicated on a nominally positive risk free rate. If the nominal time value of money is strictly zero, a lot of things start to break down.