The merger website says that the 717s will be retained - so WN will no longer be a single-fleet type airline. WN just bought 86 100-seaters.
Post 9/11 Airline is a vastly different scene. The old model of single aircraft, flying to out outlining cities worked wonders for WN. The new generation of LCC's knew they couldn't compete in WN's backyard, so they hit the east coast(AirTran), Big Cities,(Jetblue in JFK) and it made the "old" legacies turn more and more into Southwestesque.
For years, Gary Kelly has been hammered for being too conservative and Wall Street saw that they were not the aggressive little airline, (or that was the perception) and put their money elsewhere.
I personally liked what GK was doing, letting the dust settle and keeping a ton of cash around. He's been able to see that Jetblu has been able to intergrate 2 types of aircraft and expand outside the US Boarders. AirTran has been able to do the same. Look at the US/HP merger and he learned, along with every other CEO on how not to merge two airlines.
From what I've read, AAI's CEO clearly understood the landscape and wanted a dance partner. There will be 2 or 3 Legacy Carrier's and about 2 Giant LCC's. From the inital reports, Wall Street likes the deal. GK is not HDK and I wouldn't be surprised if WN somehow intergrates the concept of a row or two of Larger Business seats on all their aircraft. I've flown on AAI"s biz seat and it's not a bad deal at their price.
Just for FUN, I would move 15-20 of those 717's to the Hawaiian Islands and see how long go! stays around.