I used the dowels simply so that the parts would stack flat. Also, the top and seats are "reversible" (I have built a table with a felt top on one side to play cards and a formica on the other for meals/messes.) Cleats would work fine as would routing out slots in the ply as Ward did
. The dowels are essentially there just to locate the table top and seats so they don't slide around. On all of the tables I've built, I've only seen one broken dowel and that was by a helper that lifted one side of the table top and twisted the dowel in two on the other side (I'm not sure it wasn't intentional.) There was no damage to the ply on either side, but the dowel had to be drilled out and replaced.