Aside from generally being more cheaply made, it doesn't seem like they have to be. Steel's steel, right? What am I missing in the equation? I'm thinking about light bars particularly. Instead of milling out a solid bar and filling it with epoxy, why not a slotted core leaving the holes unfilled, and covering them with the sideplates? Seems obvious, so there must be a reason it isn't done that way; on pro bars anyhow.