"I was just going through the same in my head. I am looking at a 50F Prodigy quilt. Would love to have one and I think they are a really great value for the money. Still, what you say is also true. I already have an Atom SV, I already have power stretch fleece tights. No need to buy anything. If I take my Cocoon silk mummy liner into my breathable bivy I could bring my clothing for any extra warmth I need. Not as light in the end perhaps, but would work nonetheless.
Still, the quilt is calling though."
Literally my exact idea. Silk liner, breathable bivy, warm clothes.