The wide quilt back fabrics are not the same type of fabric as "normal" quilting fabrics 45" wide. You only have to compare the backing made to go with a fabric series with the other fabrics in the series (45" wide) to see the different thread count and quality. The backings also tend to stretch more than 45" wide finer weaves, so I would not recommend using them for major structural elements such as sashings or borders unless you are VERY careful when stitching.
If you consider that 45" fabris is, say, $8.50 per yard and
108" wide backing is about $12 per yard, price alone should be a clear indicator that there is a difference in the quality of the fabrics.
The backings I have purchased have all stretched some, but they have all washed well too, they didn't require seams in the backing and they were way cheaper than pieced 45" fabrics. As always there are pros and cons - weight it up and decide for each project what you want or need.
Personally I would use a wide back on a DWR if only to avoid the seam in the backing.