MD90 ownership costs are very low, DL bought a bunch of them dirt cheap for cash on the used market.

While their fuel burn isn't good, oil prices have to be very high (~$200) before the low ownership costs become meaningless. Also maintenance costs are higher, but DL's efficient in-house maintenance unit helps.

The benefit of cheap fully-paid for planes is they are good to pull down capacity in slow days/months/seasons, no leases to be paid. Just park them.