From what I understand from the advertising that goes with that frame, the main purpose of the batting rail is to hold the batting that you're using for the quilt on the frame. (This keeps the batting from draping on the floor, if that is a problem for you.) I'm sure the rail will hold enough batting for the largest quilt you could put on the frame.
That said, I know that some people put a rail under their frame for the purpose of holding a full roll of batting, just as a way of storing their batting. In this case I guess they put the rail through the center of the roll, though I'm not sure how that works - maybe they have to wrap the batting around the rail? It doesn't seem like a good storage option to me, since it's low, would be time consuming to wrap, and hard to get to later on.
I haven't felt the need for any kind of support for my batting while quilting. My floor is carpeted, clean, and no pets to make themselves at home on the batting. However some longarms are in areas where the floor is not easily kept as clean (such as part of a garage) or their pets get into the batting. (I think pets would get into the batting on the batting rail, too.) There is another option, and that's building a batting cage to hold the batting while you quilt. If you're interested in the directions for that, pm me and I'll try to find the site with the directions. They're written by another Innova owner, and I'm not sure they're accessible unless you belong to one of the Innova yahoo groups.