This is one of those debates where there really is no one right answer. In a perfect world (where we do not continually fly with new copilots) arm is a useful safety backup. Ironically my experience with the new guys is that they tend to get confused when the autothrottle kicks in in gusty conditions and the approach can then quickly become unstable. As pilot monitoring you are one step further removed as your hands are not on the throttles. Sods law is you will be switching to tower look up and see the speed all over the place. In Denti's Air Berlin (who of course no longer operate the 737) the level of experience in the right seat was actually a lot higher than somewhere like Ryanair. So it really is horses for courses and this is just my personal preference having tried both ways over many years.