Actually it's no longer just for twins. It's now officially Extended Range Operations. There are routes (polar) that the A340 flies that have to meet ETOPS criteria due to lack of suitable alternates.
SWA may be doing this to go over the gulf from SoCal or PHX to south Florida but that would be pretty costly for just a few routes. It would be fun to go see the water training with the SWA F/As, though!