Helicopter crews hadn't and haven't high casualtiy rates in real war. Otherwise it would be a catastrophe because helis are not only quite expensive but require expert personal for use. You cannot substitute pilots with the guy from the street. It would be absolutely impossible for such an organisation like the BoS to build, maintain and fight with the plentitude of vertibirds as in the game. And if vertibirds were that delicate as ingame in the real world, they would be used for transportation and perhaps far distance fighting, not to fly directly into enemy gunfire.
I use a mod to make the vertibirds more durable and increase their firepower considerably. Which makes me a bit uncomfortable because I plan to destroy the BoS later. 