With flames shooting up the side of the fuselage passengers will not stay in their seats until told what to do. That's why I tell the senior cabin crewmember, under compelling circumstances, to initiate evacuation as soon as the airplane has come to a full stop and advise us. From the upper deck cockpit, the crew is not in a position to instantly assess the severity of the smoke/fire downstairs. Precious seconds would be lost for the cabin crew first to describe the fiery situation and then for the captain to think and to make an evac decision; besides, pax would already be unglued from their seats. Cabin crews are trained to make the distinction between localized flames from an engine fire and smoke/flames from a catastrophic airframe/undercarriage fire.