Ahhhhhh, the beauty of the laser.
Since there is no bottom spar, not real sure how much strength it adds from the standpoint of web. On these small models I like to use it mainly because I notch it and the rib so that they interlock. Once that is glued, I'm very sure it is much stronger than just a top spar. As for the strength of sheer webbing on a wing. It adds great strength. My grain on these is horizontal. On actual sheer webbing, it is vertical. If you want to get an idea of how strong it is in the building world, just look at built up beams for house construction. I have seen 2 - 2 x 4's with a ply wood web span great distances (like over a garage or basement) that would have required a 2 x 12 or greater. As for the cross grain on the stab & rudder, yes they do help keep them from warping. Thanks.
John