Having a "backpack" for each person is silly. The device should be attached to the wall next to the window, so it will be there when somebody needs it, and it should be possible for several people to use it, one after the other. (I have lived in a building that had such a device installed, though I don't remember exactly how it worked.)
It uses a preinstalled anchor point which would not be something big, exposed to the elements. One can use multiple anchor points. It will be difficult to mount something or buckle up if it is hanging outside the window.
I'd worry about the anchor part since there is a chance you can miscalculate the length and get stuck in mid-decent. Meanwhile, anchor could get more damage if not planned properly.
