Runway vacated - you just forgot to mention dealing with the crap onboard IFE system, placating premium pax who have been mishandled by BA, placating many non premium pax mishandled by BA, help in the CW service, etc, etc. And thats just the first few hours of EVERY flight.
Oh come on CFC, that is achieved by a working member of CC at nearly every other airline in the world (including BA at LGW) to say those duties require the presence of someone soley able to deal with them is just not tenible in today's aviation world. The guys and girls manage at my current airline without having an administrator under the stairs. Sergeant Major needed putting to bed a long time ago.
The CSD is a very useful member of crew if they choose to be and, in fact, when they do choose to be they could quite easily replace another crew member in the service.