Generally, I find Frontier's Classic Plus overpriced or not really offering much benefit when Frontier is directly competing on a route against Southwest.

For example on a route where Frontier competes against another carrier (but not Southwest), Frontier is selling PHL-CLT for $39 on it's Economy fare, so $76.50 roundtrip, on many dates in April. The Classic Plus fare is $154 one-way, and thus $306.50 round-trip.

If I was doubtful that I can make the trip, I would just wait until I was close to 100% sure and then book. The last minute fares on Frontier might be still cheaper than the quoted refundable $306.50 roundtrip on that route. For $450-500 r/t I can book a last minute Anytime or Business Select on BWI-CLT. If fares were too high on other carriers and Frontier, I'd just drive.

It's also a hassle changing tickets on Frontier, with their website problematic and customer hold times sometimes long. With southwest.com, it's very easy to change or cancel.

On PHL-ATL, it directly competes with Southwest. Granted, Frontier's refundable fare is lower (and priced at $268) than Southwest's refundable fare, where Southwest prices very high. But Southwest is offering $69 WGAs if booked in advance.

If one travels atleast twice a year by flight, Southwest's WGA should really be compared not it's AT. While refunds aren't given, the funds can go for another flight without penalty. I also think in general that the points accumulated from flying Southwest have more worth than the points accumulated when flying most other carriers.

I fly Spirit and Frontier on regular Economy fare at times, when Southwest WGA lowest fares are too high. If I book with another carrier than Southwest, I'm usually 85-100% confident that I can make the flight and not need to change it.