are you truly kidding? what kind of mechanical do you envision that you can't tow a plane off the gate?
DL has line maintenance in dozens of cities around the world and contracts with other maintenance sources where DL doesn't have maintenance. that is no different from what any other airline does, including WN.
The most likely reason why DL would NOT need WN maintenance is because the 717s are very reliable and because DL Tech Ops keeps them that way.
Given that DL's system completion factor is one of the highest of any airline not just in the US but also in the world, WN will starve if they are waiting for the opportunity to work on DL's aircraft.
for the most recent DOT report, DL operated 99.8% of its flights or cxld just 0.2% of its flights. Southwest cancelled 1.1% same as UA and AA cxld 1.9%
Further, DL's on-time was one of the highest in the industry, nearly 7 points higher than the industry and 12 whole points higher than WN.