The X is to hold the batting in place so it doesn't shift and deteriorate when the quilt is washed. Flannel is a woven fabric so there is no need to hold it together in washing since the weave already does the holding together for you. Batting on the other hand is not a woven product therefore you have to stitch it down every so many inches otherwise it will come apart when washed. My sister-in-law made a quilt that she did not follow the guidelines on how close the stitching needed to be when she quilted it and when the quilt was washed what a mess of lumps and bumps as the batting had pulled apart and shifted. She was no longer around and it was a favorite quilt of my brother-in-law so I took it all apart and put a new batting and re-quilted it for him. What a job! There is good reason why they tell you on the batting package how frequently it has to be tied or sewn in place.