Hi,
I don't think there is anything wrong with using flannel on the back of a quilt with or without batting. I make a lot of Linus quilts and lap robes for the military hospitals etc. Putting flannel on the back makes them cuddly and keeps them from sliding around so much. Many of the amputees just want something to hide their injuries from their visitors who have a hard time coping. A heavy quilt would be uncomfortable. While not in Florida it gets hot and humid in the summer time in Maryland. While I appreciate the weight of a regular quilt on my bed in the winter, it would be a real burden on a hot steamy night.
If your qult is not going to have heavy quilting prewashing the back is a must.