Having used quilts for the past several years. I really appreciate the flexibility and weight savings. The Sierra Designs backcountry bed doesn't look as flexible and weighs more than a comparable temperature rated quilt. I haven't and don't know anyone who has a bed. So my opinion is biased.
I have the BC bed and love it. It's not the lightest at 3# but I'm a stomach & side sleeper and it's perfectly suited for it. The tongue is crazy comfortable to use. Never had a quilt but will give it a shot when I run across a crazy deal.
I think the BCB would be an awesome car camping/basecamp "bag". I have only crawled into one in a store but I did like the way it felt more like actually being tucked into a bed. All that said, I don't think I would be carrying one in any distance at all. I love my quilt(s) and don't see any reason to go back to anything else.