I can see why charging for carry-on bags might sound like a good idea in a boardroom. Airlines subsidize ticket sales by carrying freight for shipping companies. If the carrier can decrease the amount of weight that passengers bring onboard it can increase revenue for the flight. First airlines decreased the maximum weight limit for bags from 75 pounds to 50 pounds, essentially reclaiming 25 pounds worth of freight revenue per bag. Then, most airlines started charging for the second bag, reclaiming revenue with the charge or with the 50 extra pounds of freight they can carry. Now (with the exception of Southwest, and maybe a couple of others) they charge for all checked bags---a move they hope will pack even more freight into the aircraft.
The airlines will argue that if they can carry more freight they can reduce fairs for travelers. Maybe, although I haven't seen it yet. And don't expect the fairs to decrease if fuel prices decrease. Bags won't "fly free" on most airlines anytime soon.
Here's where the argument falls apart. Those who travel semi-regularly know that checking a bag is a pain. In some airports (at least in Providence, RI until recently) you have to wait until the bag clears the X-ray machine before you can even head over to security. Then, you have to wait for the bag on the carousel for 20 minutes until someone realizes that the sign above the carousel is wrong and your bag is actually three carousels down, and has been for 10 minutes (I'm looking at you, Salt Lake City). So, for a quick trip, it's much easier to throw everything in the carry-on, skip the hassle, and be on your way.
For the frequent traveler, the "checked luggage tax" is a non-issue. It's somewhat annoying when travelling with the family---especially with little children---but semi-tolerable. In fact, it may even allow business travelers a slight feeling of elitism.
Not so with the "carry-on tax." This is nothing more than deceptive pricing. What Spirit is saying is that they need to increase their fairs and they aren't man enough to do it. Instead, they are adding a fee that almost everyone will have to pay. I'd like them to explain to me how I can go to a meeting or conference for single night without a change of clothes or toiletries. A carry-on isn't a luxury---like in-flight TV, free drinks, or a snack---it's an essential part of travelling.
The take-away here isn't that Spirit is struggling---struggling is par for the course in the airline industry---it's that they think that by increasing their fares $45 and then calling it a carry-on fee people will fall for it. Man up Spirit. We're sorry that you're struggling financially. Believe me, those of us buying tickets really are. But call a spade a spade. We aren't stupid.