NOT to argue or disagree with Lori here, just a somewhat different way of looking at the same question --- my immediate response was "you're not missing anything". I.e., when zipped up in mummy mode, I'm convinced that for many (most?) people you are going to get more overall warmth relative to weight carried with a zipped up bag. I, for one, don't remain perfectly still once I'm asleep.

My first down bag was a WM Summerlite, and then the Ultralite followed as a later purchase. The Summerlite was (at the time at least) the lightest (warmth for weight) option that had a full zipper --- so that indeed I can open it up fully to use in quilt mode. It's wonderful, I think, to have a light, warm, full-zip bag that can vary from fully open "quilt mode" to fully zipped mummy mode, where just your nose and mouth are exposed on the colder nights.

To be clear, I don't mean to knock quilts, I just prefer the flexibility of moving from one setup to the other as conditions warrant. At one point I seriously considered getting a JRB No Sniveler or similar on the theory that I might then leave my down jacket at home, but what stopped me was the realization of how often on colder nights I do all my camp chores with my lower body in the sleeping bag and my upper body out of the bag --- but wearing a down jacket or parka.