I'm with you, Lynn. They're the exact same thing, except one has a zipper and another has understraps. Maybe a little less material and less weight for some quilts, but in trade you might have to carry a bivy sack.
Doesn't have anything to do with cost, either. Like you pointed out, quilts can easily rival the best bags for price.
As it happens, I built a 14-ounce bag a while ago. It's good for about freezing. I use it like a quilt with the zipper opened up, or if it gets cold I can zip myself well into it, no drafts, no need for a bivy sack. Gotta say, I haven't come across many quilts as light as my bag for the same temp rating (but yes, there are definitely some, and even some lighter).
It boils down to a personal preference. If someone prefers a quilt to a bag, fine. I don't see how it's possible to argue that a quilt is more versatile since it is inherently more limited (open winter bivy in a quilt, no bivy sack anyone?). I just wish that some people could be a little more objective. When we make recommendations for people, nothing's an "all or nothing" decision, really. Quilts are not necessarily the promised land. Then again, bags might not be, either. It all depends. Ya know?