There is strength in the stamped sheet metal parts that form the body. Just like a simple bead in a piece of aluminum makes it stiffer, the piece of sheet metal that is stamped to form the floor becomes much stiffer. By attaching and/or tying the cage and frame ties into the body structure as much as possible, and in as many areas as possible, you are creating a much stiffer assembly. A frame tie attached only in the small areas at the front and rear ends of the tube will still flex the sheet metal in the area where it's welded. Attach it to a substantial part of the floor in the center of the tubing, and you increase the rigidity of both. One can't move without the other moving also.