ATL-PHL may be full, but there are plenty of routes that are not. I have been on near empty flights LGA-CVG, SFO-JFK, JFK-LAX, CVG-ORD. Even flights in/out of SLC during the olympics had plenty of empties. Also, remember thursday night is a popular night for those who leave monday AM. Try going somewhere in the middle of the day on a Wednesday, you most likely will not see the same traffic. That said, why they give you a 727 when I've sat on empty 763 (domestics) I can't even begin to explain.
But at least you get the modern interiors that DL's 727s are known for. Nothing like fine woodwork!