For the current $15 fares, you don't need a membership -- just a code that's available on their website.

The WSJ has a piece about Frontier's latest strategy. I'd guess that 2015 will be less profitable for them than 2014, but with fuel so cheap, perhaps they can survive the competitive onslaught. That said, logic would suggest they'd do better flying routes that don't directly antagonize the Big Four.

http://www.wsj.com/articles/a-new-ch...oom-1440459879