The "not designed for dead-stick flight" is a bit off. Of course they are not designed to be gliders, but they are designed to be efficient, which means they have a reasonably high lift/drag ratio (in the range of 15–20 for most airliners). They glide quite well.
Indeed, it suprises people that light aircraft generally have a glide ratio comparable with a brick, whilst airliners are generally quite efficient given the incredible size.
