Although I use a sleeping quilt the subject still intersts me. When I had a sleeping bag it worked well some nights (when it was cooler or cold) and on other nights I could be steaming hot. My bag had a full length zip so I could adapt at will but I still had to carry it on my back.
I have never really understood how these bags work, temperture wise, and I know even with my quilt on some nights I can get chilly, I've only once been cold. I've always thought that to be versatile I bought one which I guessed was for the warmish conditions I tend to camp in, spring, summer and autumn mainly. I chose OK, to be more correct I guessed OK.
To stay warm in other weathers I've always assumed that I should either buy a thin but warm down 'comforter', or do I mean just a simple down blanket, not sure what the correct word is? It can either be thrown over my quilt, or perhaps I could stuff it between my body and my quilt as an effort to stay warm. The second option, which I chose for cold nights was to wear my warm jacket over my sleeping T shirt. I have two jackets, both thin and I just use them as part of my layering system. I've never had to wear both at the same time when sleeping, but I would do if it was cold enough.
For me its all about weight as well, so being able to use my jackets for both day and night seemed a sensible solution. I've found that by thinking in this way and putting it into practice it has stopped me from thinking that I bought the wrong warmth rating for my quilt.
The manufacturers lead us by the nose, so are terms like 'four season', 'down to -10 degrees' and other terminology part of their marketing technique to buy their products and to stop us thinking for ourselves?