Why do people fly WN? They're CHEAPER.... period!
Sure, there's other secondary reasons, but the fare is the winner. Of course, with that cheap fare comes trade-offs: Waiting in line forever just to avoid a middle seat, cattle-call boarding, no FC, no real alliance with other airlines, no stand-by, no meals, corny FA's, etc etc. BUT, there's that low low price... and for many, that's all that counts.
These trade-offs are also why numerous people absolutely refuse to fly WN. Same reason both Ford and BMW exist.
The Rapid Rewards on WN have some plusses... except the expiration date!! And you can't use them on anything but WN, which also means no international flights.
I'd say the cattle-call loading and no stand-by policy are the biggest reasons I don't fly WN... guess I value my time too much. I usually arrive to the airport 35-40 minutes before my flight, which is a guarantee for a middle seat on WN, versus my reserved seat on any other airline. Also, if meetings end early, or I just plain change my mind, I can grab and earlier/later flight no problem. I can also buy the lower fare for a different flight time, and then stand-by for the one I want... which so far, I've always gotten a seat.
I'll happily pay the little extra to not stand around all day at the airport just for the opportunity to say "Mooo". I just have better things to do. My time is money... so I probably come out ahead in the long run not wasting time at the airport.