While RP4242 makes some good points (the validity of which I can't corroborate), I feel like I can say without any reservations that the benefit to DAL taking the 717 outweigh the negatives to SWA in giving them up.

DAL is using them to replace DC-9s! Pretty much anything would be more economical to operate. The 717's cut CASM in half for DAL (compared to the DC-9), while SWA can operate a 737-800 for roughly the same cost.

A gain for DAL is NOT the same as a loss for SWA.

NO ONE at ATN thought the 717s were going to be around long-term (5 years). If you did, you just weren't paying attention.