What prompted me to try out a quilt was the fact that I don't sleep on my back with my arms at my side anymore making the hood of a mummy much less useful to me. With a quilt I have no zipper problems, can toss and turn to my hearts content and regulate temps better. It is also much easier to move around in and throw on or off.
As a trade off you will have to wear some type of hat or head insulation for cold temps, deal with drafts on occasion, and be much more limited in your purchasing choices.
I have entirely converted to quilts but have yet to try the concept at sub zero temps, something that I don't do very often anymore, but my summer quilt (a converted WM Highlite @ 14oz and good to freezing with base layer and hat), can fit inside my new 3 Season quilt (Katabatic Sawatch @ 23.5oz), which gives me a great deal of loft for 37.5oz.