It is a very nice idea, however you have to remember that most airlines use their own checkin/departure system to process the passengers. It is just not possible to check in all flight departures on one desk if say there are 20 differant airlines all with their own system, the agent on the desk will be spending ages flicking from one system to another, then there is the case of product knowledge, a hell of a lot for one staff member to remember if checking in 20 differant carriers.