If you consider the design of trusses used in a bridge; or, the trusses used in the roof support of a building, it becomes easier to understand the correlation with placing diagonal shear webs in a wing spar assembly.   In full scale (which Rodney is referencing) the additional weight of building spars in this way can easily be absorbed.  It may be that a spar built in this way might work for smaller models, also.  However, my opinion is that vertical webbing works well enough, why go to the extra effort?