From a coach standpoint Song was meant to fight B6...clearly it has not worked as intended. With DALPA still shaking down DL on these flights it is debatable how much is being 'saved' on operating these flights vs. just operating them as mainline. Same FA's, same ground handling, same rez systems.
The only thing that is being saved is no F, but from what I gather from the folks that are still flying this carrier, that is even less than it used to be also.