A bag rated at 0 C is a good all around bag. We have used ours in temperatures ranging from 43 C to a little lower than 0 C., all on the same tour. As Machka suggested, just sleep on top during warm weather. Add a little more clothing if the temperature really drops.
I also think synthetic bags are the way to go. The newer insulating materials compress as well, and are as light as down, they are easy to clean, dry quickly, and they are do provide some warmth when wet. I can tell you from experience a wet down bag is next useless.
We were in a campground, and I had my synthetic was draped over a picnic table to air out. A dog came by and urinated on it; I guess just because he could. The campground had laundry facilities, and in a little more than an hour my bag was clean and dry.