I don't understand why anyone would use a federated flight search when looking for a domestic flight in the US. None of the major airlines come close to Southwest in terms of price or hassle free travel and none of the flight search engines include price information for Southwest.
Perhaps for the still-common case of flying to an airport where Southwest doesn't go? Southwest gets to be cheap in significant part by only flying routes with enough demand to fill a 737.
And on rare occasions another airline does beat Southwest's price. Sometimes I see flights in and out of JFK turn up cheaper on Delta.
