My assumption on the design of the malt pipe is simply to keep it from being a mess during draining/sparging. With holes in the side, if you had it resting up on its posts, you'd probably get water coming out the side holes which would then run down the outside of your brew unit, making a mess. If you note the Anvil Foundry 10.5, its malt pipe has holes on the side, but they are strategically placed at a level under the feet of the malt pipe. So when you pull it up to rest during drain/sparge, anything that comes out of those side holes is captured within the body of the brew unit.
Plus it's easier to manufacture a sheet of steel rolled into a cylinder and welded than trying to roll a perforated sheet, or trying to drill a bunch of holes in a solid sheet after you roll it.