On some aircraft, not all by any means, but depending on the mod state, each FCC can either provide an autopilot in command, or a flight director. For example, on some of the aircraft I fly when all 3 autopilots kick in on an ILS approach you simultaneously lose the FDs on both sides. So using centre command saves faffing about with the flight director source selector...
Not saying that this is necessarily the reason why (maybe some people just like to keep things symmetrical!
) but it's one possibility that occured...