The other obvious point about having 2 pilots is that sometimes you have to hover left side on to the obstacles/deck etc to get the job done and the LHS pilot sometimes has the best references.
One other difference between Mk 5 and 3/3A is the RN LHS pilots don't have access to the nav kit (CDNU) - this is in the back with the observer whereas the Mk 3/3A have the CDNU (RNAV on 3A) in the cockpit. On the 3A, the RNAV can drive the aircraft around search patterns and nav routes as the SN500 FPC has autopilot modes and a dual channel ASE. Radar waypoints can be sent forward from the Radop to the RNAV saving a lot of time when search planning and allowing a cross-check between co-pilot and Radop when plotting grids and lat/longs.
The SN500 FPC is vastly superior to the Mk 31 FCS and doesn't have any nasty failure modes that try to fly you into the sea; instead, the performance monitors and the duplex ASE do a pretty sterling job along with the extra facilities (rad alt on manouevre below 1000 ft being one of the best).
The RN allegedly pulled out of a plan to put SN500 in all Sea Kings because they knew they were getting the Merlin.