Thanks for advice Drew. There are a few reasons for the way that this process is done in this particular way. The First is the cross pieces makes sure that when you adhere the top and bottom panels in the vacuum bag, the top and bottom to not become wavy when the air is sucked out of the bag. The second is that it adds more glue surface. Finally, in my opinion it feels more solid. You can knock on the piece and it doesn't sound hollow. The problem with using solid wood as apposed to MDF it has a tendency to bow. If you glue a bowed board for the long perimeter, you will then need to re-square the whole panel. With MDF you can make the perimeter to final length and width and flush trim the top and bottom pieces. I would use a different material for the perimeter if for the joinery you plan on is putting dominos or screws in it. I think you are seeing this process only through the eyes of strength.