[Note: the following is merely my own personal experience and musings on the subject at hand. It is NOT intended to disparage anyone else's experience. In fact, since quilts are so popular with some many very experienced UL-ers, I have to say i must be missing something in my own personal objections based upon my VERY limited experience in this area. So, please read the following with this background in mind.]
I've tried the quilt approach in other than hot summer nighttime temps and didn't like it due to the entry of cold air upon movement. Reheating that air with body heat and valuable calories just doesn't seem to make sense to me.
I need to make clear that i sleep like a dead man, in other words, i rarely move when sleeping (on my back), maybe just 1-2 time per night a minor readjustment is needed to restore some circulation to areas that had been under the pressure of body weight.
Now, i don't use top-bags either, but that's just because i'm unfamiliar with any but the GG, BA, and some SD ones - none of which seemed to meet my needs at the time a few yrs ago when i was beginning to lighten up.
So, why don't quilt users use top bags in colder weather? Is it that the top bags available just don't meet their needs (fit, temp rating, no zipper, need for a special sleeping pad, etc)? Or, is it that last couple of tenths of an ounce (or a bit more???) in weight that is being saved by eliminating the bottom fabric? Does it really save ANY weight at all, since the quilt needs to be made wider to avoid drafts (plus you might want straps in colder weather)? Does all that extra top and bottom WIDTH fabric weigh (plus straps) less than a zipper? [probably?] What about a zipperless top bag (GG sold, or still sells one)?
I'm NOT talkin' warm or hot summer night here. I can buy into a quilt for those sleeping temps where on might prefer to throw the quilt off (just like blankets at home) for a portion of the night when one gets too hot, and then cover up if one gets too cold. Sure i can do it with a mummy bag, but, i'll admit, it's easier to do that w/a quilt.
Also, the lack of an integrated hood (in other than perhaps Dr. Caffin's homemade quilt) also, to my way of thinking and very limited experience, is a weak point also. It's fairly easy for movements to allow cold air to enter at this point. One thing i really like about a mummy bag is the integrated, very well insulated hood. If my head is kept very warm, then the rest of me stands a better chance of stayin' warm too, all other things considered equal. For me, personally, this is very impt in COLD weather. I know that there are down insulated balaclava's/hoods/headgear that some "Quilt-ers" will use in cold weather. However, doesn't this still provide an entry point for cold air b/t the head-covering and the quilt-proper?
Plus, in COLD weather, a 4.6oz (or heavier - up to maybe 7oz) Cuben fiber or PQ + nylon bivy is added to help eliminate drafts, but still allows for some air xchange b/t quilt & uninsulated bivy which, again, needs reheating by the body. Isn't any weight savings of the quilt is lost if a bivy is used?
In cold weather, i swear by a mummy bag with the opening nearly totally closed up with, usually, only my nose sticking out just a wee bit, & an OR gorilla or Seirus neoprene full-face balaclava covering my nose to keep my "snorkel" warm also.
Ok, i know i'm not the sharpest tool in the shed, so what did i not understand in my quilt experiments? What did i miss that has caused me to stick with mummy bags?
Many thanks in advance to those who take the time to educate me with their replies.