Actually, laying a two by four flat weakens the wall strength. Sure it is thinner, but think in terms of this, lay a two by four between two bricks, stand on the 3-1/2" wide side and it will probably touch the ground, now stand it up and it will hardly bend. It also allows more contact area between your walls, especially if you connect them the way we do which is just butting two walls up against each other and running 3 2-1/2" screws in the 3-1/2" side of the two by four of one wall in to the next wall. We also brace across the tops of our walls heavily. We have very destructive customers because of where we are located, so I never build with anything less than 4 two by fours per wall with 1/2" sheet of plywood on at least one side depending on where it goes. We have some walls with 3/4" plywood and built out of 5-6 two by fours where the customers are most likely to run into them. More than a few times we have had 1000 pounds of customers try to run through the walls instead down the hall way. I can post pictures if nessecary. Our haunt consists of approximately 1300 of these wall panels, some of which have lasted for upwards of 15 years.