Hello,
If a bench, (or bridge or pylon or tower) is built properly, it WILL include cross bracing, whether it looks like it has it or not, if a structure is to use the minimum amount of material for the strongest outcome, the triangle will be incorporated somewhere. It is a basic engineering principal. If you are going to go to the trouble of building a bench, leaving them out is likely to lead to disappointment, and for the sake of what, exactly? A couple of bits of wood. Otherwise fit a board over the entire back, which amounts to the same thing as a brace. You can link as many rectangles together as you like, but you will not stop racking, unless, of course, you build a bench like mine, which is so massive, with enormous pegged through tenons, it doesn't matter. For a small, lightweight bench, the diagram I posted is perfect.
Mike.