Someone already beat me to it, but I believe our practice of assuming BG's soak up bullets and we go down after 1 is just looking at the worst possible outcome.
You don't want to depend on one bullet putting down the BG.
You don't want to depend on still being in the fight after you take one.
You should hope for the best, but plan for the worst.
For the opposing viewpoint (bullet proof good guys and bad guys that can't shoot and go down easy) you can watch pretty much any action movie.
si vis pacem para bellum