The quilt has less down or no down at the bottom, because the fact that down provides warmth by trapping air, the down at the bottom is compressed by our body when we sleep in it, the compressed down squeeze the air out, thus does not provide the same warmth as other areas with loft.
As a result, it is easy to understand from UL backpacker perspective that nixing the bottom down, fabric and zipper saves weight, also given the fact that most users are already leveraging a sleeping pad to insulate the bottom.
While some Alpine manufactures are using variable fill/deferential fill, they purposely fill more down at the bottom to provide more insulation, because it is touching the cold ground and compressed.
As Alpine mountaineers/climbers may not using any sort of mat to insulate the bottom and solely rely on the bottom of the sleeping bag, is it right? If yes, given the fact that the compressed down does not insulate well, why compensating this by filling more down at the bottom instead of just use some sorta light weight sleeping pad? If no, they are using pad like backpackers, then still why filling more down at the bottom, it is not very effective anyway, and the weight saving is also crucial for mountaineers/climbers, because apparently they have more gears to carry than backpackers. Is there a contradictory here?
Another question is whether the down at the bottom really a waste/dead weight? I doubt it, because in reality, only the down completely under your body are compressed, all the rest areas, such as the gaps between your legs, the areas between you arms and your body, the surroundings between your neck and head etc. are all not fully compressed, I would argue for the areas under your body are not 100% compressed, assuming it does, it most likely still have more insulation power than 2 layers of fabric, and when you toss and turn, the previous compressed down regains their loft and becomes uncompressed and live again. Isn't it?
Overall the sleeping bag is enclosed chamber and prevent your body heat escaping outside. So for winter, sleeping bag is far superior than quilt in my experience. YMMV.
Maybe putting some sorta stretchy fabric like spandex to close the bottom of the quilt and making the quilt fully enclosed to provide more warmth as well as maintaining the weight saving with no down at the bottom and no zipper at the side is a good compromise?
Sorry for my rambling…
