I don't think adding crosses affects lateral stiffness much at all. What it does, I think, is add stiffness in the, I'll call it "transaxial" direction. Imagine planting the rim and torquing on the axle on an axis normal to the axle of the wheel. This is the kind of stress the wheel sees when it is turning... The axle is being torqued on an axis normal to the axle by the fork while the rim wants to roll straight. Crossed spoke patterns give the wheel stiffness in this direction and should give the wheel (we are talking about front wheels here) a better handling feeling in a corner. This, I think, comes at the expense of radial stiffness.