A Spirited debate
But let's face it: for all intents and purposes, this carry-on bag fee is MANDATORY, because you'll have to pay it for all but the shortest of trips, for which you can pack light enough to fit your bag under the seat.
I urge you to voice your opinion directly with Spirit Airlines, which I just did. Just go to their contact form, and start venting. Let's start a "spirited" debate!