This came into my mind after watching Air Crash Investigations' season 15 about the UPS6's catastrophic crash. The episode is "Fatal Delivery".
After a fire caught in the cargo of the airplane due to lithium batteries, the crew aboard the plane tried to land back in Dubai but the aircraft missed the approach at Dubai causing it to crash.
Why would we tie two crew members to the fate of tons of tin, cargo and fuselage?
First of all, ejecting off an airplane is NOT an easy thing that you can do anytime anywhere at any altitude or airspeed.
In passenger planes, this approach maybe challenging[but who knows maybe it will be the future]. In passenger planes, people who feel that this idea is helpless justify so by referring to some design specifications in the aircraft itself and also the lack of training of passengers, so who knows maybe redesigning the aircraft can allow such thing.
In cargo-planes, the situation is different, you have a small number of crew members and huge amounts of cargo with them. Imagine if there is an ejection checklist where the pilots decide that this cargo aircraft won't make it, and it is time to save their lives instead of tying it to some cargo.
Read more:
https://en.wikipedia.org/wiki/UPS_Airlines_Flight_6
http://www.bbc.com/future/story/20131223-should-planes-have-parachutes
http://aviation.stackexchange.com/questions/2186/why-are-commercial-flights-not-equipped-with-parachutes
https://www.quora.com/Why-dont-passenger-airlines-have-ejection-seats
