Some airlines, including Southwest, are also simply answering customer demand that frequent-flier miles be easier to cash in.

WSJ: Frequent-Flier Miles: Best Airlines for Cashing In

The research unearths a tip for travelers having trouble using their frequent-flier miles: It might be easier to get "free" tickets using miles on low-fare carriers than it is on big carriers with global networks.

WSJ: For Frequent Fliers, a Ranking of the Stingiest Airlines