Spirit rationalizes their new fee
"In order to continue reducing fares even further and offering customers the options of paying only for the services they want and use rather than subsidizing the choices of others, we are progressing to the next phase of unbundling with this introduction of carry-on bag fees. In addition to lowering fares even further, this will substantially reduce the number of carry-on bags, which will improve in-flight safety and efficiency by speeding up the boarding and deplaning process, all of which ultimately improve the overall customer experience."
You could fly the largest plane in the Spirit fleet through some of the holes in their logic.
First, that by bundling the "cost" of carry-on baggage into the price of a ticket, they are making you subsidize the "choices of others." As if to suggest there is a measurable percentage of people who bring only enough baggage that will fit under the seat, and who don't need to check bags or stow bags in the overhead bins.
Second, implementing this new fee will "substantially reduce the number of carry-on bags." Really? If I have to pay a fee whether I check my bag or carry it on, which option am I going to choose? I'm still going to carry on if possible, so that I don't have to wait at baggage claim.
Third, that this new fee will speed up "the boarding and deplaning process." Even if you believe that there will be fewer carry-on bags, do you really believe it will take less time to board and deplane? Perhaps, slightly—if you swallow the fewer-bags claim—but I can think of other, more customer-friendly ways to accomplish that goal than charging this new fee.
And lastly, who among you can believe that anything an airline does in this day and age will possibly "improve the overall customer experience"? That claim flies in the face of 20+ years' worth of trends to the contrary. It will take a sea change in the airline industry to start making a positive impact on customer experience, and nickel-and-diming customers with more à la carte fees is not a step in that direction.
On their website, Spirit Airlines claims to "liberate customers from being forced into paying for services they do not desire or use." In my opinion, they are not following the "spirit" of that mission by charging a fee for something that is as "optional" as a lavatory. (Knock on wood; that may be next.)