The shear webbing will increase the stiffness and strength of the wing. Many people don't understand the mechanics. The idea that the spar size needs to be increased if shear webbing is added is blatantly false.....The spars will always fracture given a certain stress. The shear webbing joins the lower and upper spar into one beam (mostly carrying a shear loading, thus shear web). The moment of inertia is greatly increased. It kind of transforms it into an I-beam. The stress caused by the bending moment is inversely proportional to the moment of inertia. Therefore, the higher the moment of inertia, the lower the stress on the spars. Just look at the web on an I-beam, or engineered lumber. It is very thin. The further out the material is from the center the higher the moment of inertia.

Basically, the shear webbing will decrease the stress on the spars given a certain loading. The wing will be stronger given the same SIZE spars. The shear webbing can be thin and very light. Like was mentioned earlier the maximum bending moment occurs at the center of the wing and shear webbing may not be needed out near the tip. The outer edge of the wing does not create the same amount of lift as the inner section. But that's another subject.