The issue I have with Spirit on this particular front is charging for carry-ons.
If the airline is charging for carry-on but offering a dirt-cheap base fare, I would not complain one bit. It's not like the other airlines are angels, they also factor in the cost of flying a carry-on baggage but tag it into the base ticket fare for everyone, whether you are taking a carry-on or not.
Look at it this way- one airline offers a $50 base fare and option to add everything as per your requirement- $20 for carry-on bags, $5 for on-board soda and peanuts, $5 for in-seat TV screen, and another airline offers $80 ticket with one carry on, soda and peanuts and a TV screen included, which is better? Neither, they are both charging the same, but in the case of former, now I have the option to take only what I want. For example, next week I am flying to Philadelphia for NTD and will have only a college backpack with me, so in that case I would prefer paying $50 base fare and declining the rest (I can keep my backpack under my seat, for which there is no charge on Spirit). For a 2 hour flight I don't care about half a can of soda and a TV screen, so why pay the extra $30 for things I don't want?
Spirit is like make your own pizza.. decide what are your requirements and pay just for that, simple. As costs go on increasing and economy continues to be the way it is now, this approach is the only one that will probably make business sense. Flying today is only a means to get from A to B, the glory days of Pan Am when flying used to be a privilege and an event to look forward to are long gone.