What thickenss plywood did you have in mind? 1/2"? 3/4"? 1"? The construction of HCD (cardboard honeycomb in between 2 thin pieces of plywood) makes it much more rigid than just plywood. On the train layouts rigidity is important (even if that means having to deal with that hollow base construction). Plus HCD is lighter than plywood (so easier to move if needed).