Thanks for the links, I'm very familiar with those articles, having studied them before my purchases and during. Your guidance through your experience is invaluable on this forum- so thanks a lot.
I use the top bag for mid summer (August) and take along the quilt as an overbag for shoulder season hikes (July and September) It is cheaper than me buying two bags, and more flexible. I have a heavier bag. a three season down bag from 20 years ago, and I've never had a comfortable night in it as it is too warm for me. The top bag is just right, and if I am wearing the quilt on top, and it is too cold- I can just roll the quilt away, or back on again if the temperatures drop. I also find that it prevents draughts and mist hitting the sleeping bag, so I can shift it to the windward side to form an insulating block. It is aslo great for sitting around camp, I can drape it over my shoulders or legs for warmth. Also, as it is synthetic, I can be confident that if something happened, and my sleep system got wet, at least I'd have something to keep me warm while the down dried out. in a word- flexibility.