I'd have to disagree. The advantage of close fitting webs between the spars is precisely that they DO NOT rely on adhesives for the shear strength. The adhesive just stops the webs from moving around, the strength is obtained because the webs are a close fit between the spars.
I have to disagree back at you...
Close fitting shear webs are fine, but with a weak adhesive bond at the edges due to a tiny bond area, any failure in the adhesive to a spar would allow the shear web to collapse. Shear webs require complete constraint of the web to be effective, which I would argue is more difficult to achieve in practice with an assembled I-beam.
FWIW, most adhesives are actually very strong in shear...they are generally weaker in tension and poor in bending/peel. This is why scarf joints are used to joint two flat plates, to maximize the shearing forces in the adhesive joint which the adhesive is better equipped to withstand. Similarly, the vastly increased surface area of adhesive as in a C-spar allows a smaller shear web to be used (if desired) and I think have a greater degree of "operator independence".
MinnFlyer, Volfy is correct...shear stress does decrease in a cantilever arm which a wing half approximates. Point loads or distributed loads, doesn't matter. The part of the equation your interpretation is missing is the sum of the moments along the beam. For example, using your illustration the bending moments are maximum on the right end of the spar and zero at the wingtip due to the point load on the left end. At any infinitessimal point on a beam in equilibrium, the sum of the forces and moments must equal 0.