JetBlue has lucrative NYC/Boston slots, very profitable Caribbean routes, a solid business traveler following, innovative in-house technology, and is the first to serve Cuba. They've also got incredibly good codeshare agreements lined up with Lufthansa and Emirates, expanding their reach to the entire globe. All those things are valuable regardless of the different fleet type.

Even though their first go at Frontier didn't succeed the fact that F9 operates Airbus made no difference. During that time the old Frontier was going to allow SWA to quickly dominate the DEN market and increase ticket yields dramatically by going from three major DEN airlines to two. The Frontier of today puts pressure on other airline fares and as it expands will be a major threat to future revenue earnings at carriers like SWA. Buying Frontier eliminates that threat and secures additional marketshare. The Airbus fleet could be phased out and the orders could be sold/leased to another carrier for a premium or even cancelled. That'd make Boeing pleased and would probably yield even better deals on future 737 orders. If SWA could find someone to lease all those AirTran 717s to I'm certain they'll be no shortage of airlines lining up for new Airbus.

I doubt SWA is actually looking at either carrier, but stranger things have happened. They bought Airtran for a lot of the same reasons. AirTran had far lower internal costs and were a major threat to SWA's RASM if they continued to expand (and they were well on their way). Not to mention the ATL flying and international routes. It's not about fleet types and products. Those variables don't take long at all to adjust. It's about eliminating competition and gaining marketshare/dominance for them.

In any event, this is all wild speculation driven from an article where just a few months ago the author claimed SWA was targeting entirely different carriers for another merger. Don't go seek a job at F9 with the idea that you'll be a SWA pilot.