rubik101 - The flight deck was subject to different forces to the passenger cabin - came down from a higher angle, stopped in a shorter distance - so you cannot compare the two. I also imagine that if they just had a lap belt then a sudden stop would result in a lot more injury from contact with the various panels, etc. around them.
If I were to find myself in an accident and had the choice I would go for the 5 point harness (or at least a 3 point one) every single time over a lap belt.