The advantage of using the FMS to do the orbit is obvious if you have a strong wind. Try doing an "orbit" using the heading bug with a 100kt crosswind blowing. You will end up at least 10nm downwind of where you started. Do another, and guess what?
The FMS will draw a symmetrical circle magenta line and adjust bank angle to make that circle, ending up where you started (well it does in my 717, which is the same FMS [visually at least] as the scarebus).