I don't know that your assessment of a quilt + beanie is more weight than a traditional bag, far from.
Golite Ultra20 Quilt = 540g
Beanie = 27g
Total = 567g
How many traditional bags with the same temp rating, and everything else being even, weigh less than 567g?
Also take into account that a Beanie can be worn while you are walking, making dinners, sitting by the camp fire, cozie for FB meals etc.