IMO-- Underline Opinion!
A sleeping bag is more versitile and comfortable than a quilt, especially in cold weather. I have been fortunate enough to use several sleeping bags of various temperature ranges, -40 F to +45F, for a number of outdoor activities. The bag I found the most useful for three seson use is a light weight synthetic bag that is rated at +25F. It is the one we go to for most extended bike trips where a wide range of temperatures are possible. For short summer tours a +45F bag is used.
On a tour this summer we encountered high temperatures of 108F and lows of 28F. The 25F rated bag worked well on this trip.
Why synthetic? It actually can keep you warm when it is wet. I know a lot of folks say that they have never had their down bags get wet. In my experience, I have not been on a tour of any length where my sleeping bag has not picked up some moisture during the trip, either from rain or condensation inside the tent. Synthetic bags can be washed safely in any laundromat. Maybe more importantly they dry quicker than down in a breeze as well as in a clothes dryer.
I don't think odor is an issue. I used my synthetic bag for a three month period, and only washed it once. The only reason that I washed it was that a dog came over and urinated on it when I left it laying on the ground while setting up camp. I rode into town, found a launromat, washed and dried it-all before dinner.
Why a bag over a quilt? I believe that a bag is more efficient at capturing and retaining body heat. Also, unless the quilt is wrapped tightly around you there is less air space inside a bag than a quilt. The less air space that needs heating the warmer it is. Sleeping bags can be significantly lighter than a wool quilt, and maybe more importantly they are a lot less bulky.
A nice sleeping bag could be purchased for the same price as a Pendelton blanket. We have a Pendelton blanket on our bed (queen size), but I really would not want to pack that thing on a bike trip. I'm not sure there is anyone in Oregon that does not have something manufactured by Pendelton. The "Oregon business suit" is a Pendelton shirt and a pair of Levis.
Having said all that- what it really boils down to is personal preference.
This is my 25F rated synthetic bag in the yellow/green compression sack--weight <2 pounds, about the size of a small loaf of bread. My tent is in the blue sack, and the Thermarest is in the orange bag.
P.S. Prathmann, My post was being edited when you posted, and I did not read your post before I posted this. I did not mean this to be a down vs.synthetic debate. I was just expressing my preference.