Hmm Lem, good points made there. I think a little bit different now
Anyway on the ground i still sincerely think that waiting for 30 secs with passengers disembarking (decision has already been made anyway) it's pointless to wait and see if after 30 sec. the fire has extenguished.
I mean if you were not able to turn the aircraft away from a prevailing crosswind, the fire leaps quikly over the wing to the cabin and to the other side of the plane.
If the fire hasn't extinguished after 30 sec when bottle 1 is fired, you're about certain that the second bottle doesn't help either. Because probably the fire is already going further than the core of the engine. I do think that a little delay is there for a reason. So wait let's say 10 sec. or something, then discharge nr2 and get the hell outta there....
And again, in the air it's completely different reasoning, and 30 sec logic is well in place there.
Good subject for debate though on the ground.