Plywood is built the way it is, fiber orientation and ply thickness, to balance off the stresses of drying/humidifying so that there is minimal warping. Wood expands/contracts with changes in humidity and it contracts at different rates on the three axis.

Structurally, the inner ply adds negligible strength -- it only separates the outer plys. Think about steel I-beams or those wooden truss beams in houses. The center web only serves to separate the flanges.

As an aside, that is why you can put all the holes you want in the middle of a beam for wires and pipes without affecting strength. A 2x10 beam loses only 1 percent of its strength if you put a 2 inch hole in the middle and 3 percent with a 3 inch hole. That is also why a 2x12 is twice as strong as a 2x10.

Back to plywood, especially three layer plywood. The top and bottom ply are oriented in the same direction and so the plywood has the same bending characteristics as the outer plys - - stiff along the grain, weak across the grain.