Airframe Cost
The largest advantage (IMO) is the initial airframe cost. Even modest total hour/cycle frames are are easily had a garage sale prices and the Dog series was made with lots or real metal, so major mods in the conversion are minimal. As others have noted, fuel burn at near MTOW will be high, but package carriers are used to that. For a specialty carrier, perhaps on a fixed schedule with light loads and small containers, I can imagine a few of them. Two hundred MD-80F conversions seems like some salesman's wet dream. (And sadly, MD-80 drivers in need of work are cheap.)