wombat: the philosophy can be tricky sometimes ..
evacuation with an engine fire...very easy to imagine one where no bottle is already fired: engine fire on takeoff.
In such a case our SOP is reject then evacuation. No time for engine fire drills before evac.
The latest Boeing QRH changed the recomandation to discharge bottles 'just in case" as I said in the post above.
Now is just override and pull. Makes sense somehow.
Brgds Alex