The spar webs improve the wings bending stiffness, not torsion. The wing skin completes the torsion box. To build in the maximum amount of stiffness, both in bending and torsion, ideally you need both. However with a wing like this, and realising the stresses are all concentrated at the middle of the wing, you can reduce the thickness of both skin and webs towards the tip. Check out the wing skinning on Tom Bretts Perigee, Tom knew what he was doing, and the skinning follows the load pattern. The bonus with the idea is that the tip weight is reduced too, so damping in roll is improved.
Evan.