I don't see where SWA is gaining by subleasing the 717
The operating costs being very similar might be a factor, when you consider the revenue potential (122 seats versus 143). Philosophically, we are going to larger airplanes, not smaller. 143 seats, -800s, ..., the 717 is like leasing a fleet of -500s.
However, one benefit for the SWA Master list (considering the transition bid results), subleasing 52 * B717 will more or less "normalize" the seniority list on 1/1/15. That is, on 1/1/15, those senior enough (≤50%) to hold a Captain seat will, those unable to hold a Captain seat will be bumped from the B717, assuming the seat removal occurs in reverse seniority.
If the B717 were all still on property on 1/1/15, we'd have close to 300 former AT pilots in the left seat, where the bottom B717 CA would be junior to over a 1400 former SWA FOs.
All considering no pilot growth.
Maybe, this was a small part of their plan to keep training costs down.
I do see a fair amount of training after 1/1/15, when former senior AT guys bid back to Captain, displacing former SWA guys back to the right seat. But who knows, maybe some of those guys will like the top bids, vacation, premium OT, .... and will stay as FOs.
IMO, the company really looked at the costs associated with operating a multiple aircraft fleet and it worried them.