2 piece full bore stainless steel ball valves provide unrestricted flow through the valve which minimises any pressure losses across the valve. This is the most economic solution to provide a full flow stainless steel ball valve.
2 Piece ball valves cannot be maintained in-line, they would have to be removed from the pipe line to carry out maintenance work. For this reason they are generally considered to be a throw-away item as opposed to a maintainable item.