As I understand things if you depart with one or more reverser's inop the FADECS are set so that if reverse thrust is selected then you don't get anything more then ground idle.
In the case of a G+Y failure the FADECs obviously haven't been changed so if you select reverse, either idle or max then you get the requested power setting but without the reverse doors! Not useful when trying to land with barely any braking or spoilers!