Maybe I'm not understanding Favorite Fabrics either or some of the other comments. Why would you want to use a wide backing fabric for piecing? That is way to much fabric to try and fold and cut for piecing. The difference in cost for 44" wide versus 108" to 120" wide should in my humble opinion be self explanatory. I believe Wilma was just trying to explain her fatbacks which are what many people consider wide backs but she/her business have named them fatbacks. She was also just trying to explain the quality/differences in them.
I don't work for Wilma but have purchased from her before and the quality is excellent.