For me, oxygen masks are there for 1 reason - a lack of breathable air in the flightdeck (decompression above FL100 or smoke).
I can see a number of examples where you can perform a rapid descent but not require oxygen masks (e.g: APU Fire, Cargo Fire, red bomb threat).
Oxygen masks severely impede cockpit and cabin communications and therefore I personally would only use them for the above mentioned flightdeck environment issues (smoke or lack of air)
Cheers
Pro Spin