SheldonYoung,

Shear webbing needs to carry a shear load ( you can think of this as a sliding load ) to prevent the top and bottom spar members from moving relative to each other in the spanwise direction. If you use an 'I-beam' configuration, you still need the web to be securely joined to the top and bottom members. Those joints require glue to carry the shear load. Actually, the lap joints you would have with the webbing glued to the back would carry less stress, since the joints would have more surface area over which to distribute the same amount of force. Generally, to have a stronger glue joint, you want a good fit, and enough surface area. It is easier to get both by lapping shear webbing to the back ( or front ) of the spar members than by edge-gluing the webbing in between.

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