have only used sleeping bags, so I am curious-re weight, wouldnt the only difference be the zipper weight? I agree that mummy bags can be constraining, but I see how opening or closing a zipper more or less is less convenient than various opening options of a quilt? Dont you find that your feet tend to stick out from moving out when asleep, or arms or whatever?
Ive never seen anyone use a quilt in my years camping, so while I know how they work at home in a bed, I do wonder about the stated advantages. I do see the disadvantage of waking up cold if your leg or whatever sticks out, or if in turning, you end up not wrapped up--more for cold nights I mean.