I confess to not knowing anything about the B737. However, I would ask the question... "How many fire bottles do you have"? The B707 had two bottles per wing and if both bottles were discharged into, say, No 1 Engine, No 2 engine would not have any fire suppression capability for the remainder of the flight. As for an engine fire on the ground, well, I would strongly support the option of throwing everything you have at the fire and getting the hell out of there... Captains decision of course!