I doubt it's got owt to do with processing hold baggage, few less pieces aint gonna make a big time saving. It's a definate attempt to attract the business type with his overnight bag and lap top. These people dislike standing in line to collect their bags and will do their level best to get hand luggage only on board. Let them know that they will not be hassled over reasonable hand luggage and they may opt to book on your flights!
Joe public generally have no need to load up their hand luggage and those that abuse will have it tagged and stuck in the hold anyway.