I'm not a part of DCI so I don't have the inside perspective but it seems like DL likes to spread all the carriers around instead of concentrating them in one or two cities. I understand that they want to avoid a management doomsday scenario like a strike or a situation where one carrier gets a large portion of their fleet grounded but is this really an efficient operation?

It'd seem like it'd be better to keep everyone in their respective hubs to maximize efficiency and then deal with the problems as they came along.