DL has been working the hell out of the 744 fleet and probably wishes it had more. Fuel consumption and refurb charges are more than balanced out by the fact that they're bought and paid for and provide welcome TPAC lift. Retirement in 2017 will mean they've given 22-28 years of service and the recent refurbs, along with C checks, etc. are just part of the predictable cost of owning / operating over that timespan. They still got their money's worth and then some.