Yes, thanks for pointing out that the insulation left off the back would have provided very little additional warmth. Better to leave it off and save on weight. Another advantage is that you do not need to sleep in a compact bag to save weight. The quilt covers what it needs to w/o limiting your range of motion. If I used a bag it would have to be large rather than compact in order to provide room for my shoulders, so compact is not an option for me.
Another weight efficient ploy to gain additional warmth is to use the sleeping quilt in conjunction with a light, breathable, water repellent bivy such as the superlight bivy made by Mountain Laurel:
http://www.mountainlaureldesigns.co...id=30&osCsid=3f134942bc4b665391a28ded6a8e7137
This bivy only weighs about 6 oz and has a bug proof face aperture for the occupant. Because the bivy material is so light you can afford to get one large enough to fit you with your quilt and ground pad.
Finally, Nunatak quilts can be obtained with ratings all the way down to 5 F.