Yeah, why would anyone want a hands-free water reservoir? Why carry water anyway? You can't make fire with it. You can't baton it through a log. It doesn't take the place of a survival gun. Seems like just a lot of extra weight if you ask me. Okay, maybe if you are in a fire prone area and want something to keep your fire from getting out of hand, then maybe.
Seriously, an extra flap of thin material and a hook doesn't really translate to a lot of weight. I'm not sure that a 2 L bladder doesn't weight less than the same amount of nalgene. Maybe you could clarify where you think the extra weight is coming from?