I have made many quilts with a flannel backing, one nice thing about it is, it helps prevent the quilt from sliding off the bed or person, the flannel seems to grip the surface. I have always used a light cotton batting with the flannel.
But I have heard of people using just the flannel and no batting for what they call a summer quilt. You may want to keep in mind the weight of the quilt on the person if they are an older person if you do use both batting and flannel.