Rusty,
I'm one who tends to err on the chilly side, especially when the temps dip below 32f, doesn't matter if I'm properly layered or not I will at some point feel "chilled", but that's just me and often its all in my head. Switching to an insulated pad has helped with warmth tremendously, as does using a bivy with my quilt and being wise in my camp selection ( obvious one). Many people use quilts well below freezing, but it does require some planning and additional steps to keep warmth locked in ( ie. bivy, balaclava, down hood). The bellows effect is an area with quilts that can be problematic, more than a sleeping bag, if you make lots of adjustments or move around ridiculously at night, its possible to expel warm air through the head end of the quilt or underside. The same can occur in a bag, particlarly through the top in this case, but is less likely, especially with a solid draft collar which most mummy bags have.
Personally, if I got out more frequently in sustained cold weather <20f, I'd use a mummy bag with a full collar, but again, I tend to run on the chilly side of the spectrum and the slightest draft gets under my skin. I won't go much below 20f in a quilt, but that's about the limits of what I currently own and rarely a low temp I see during the year.