What Southwest isn't talking about is what their cost structure does to an airplane designed and used as a regional feeder.
AirTran's RASM on routes served by the 717 are much lower than where they need to be under SWA's CASM. My guess is that SWA will pull Florida and low revenue flying out of Atlanta and go after core markets which drive higher revenues. The 717 is not suited to that kind of flying.
Head to head with a CRJ700/900, the 717 doesn't have a clear advantage.