Spirit is out front with these sorts of charges...and given their target audience and longstanding fees for everything approach, they may get away with it. This isn't a traditional carrier.
Given some of the carry ons I've seen lately, perhaps a carry-on fee without a checked bag fee (and where they really NAIL you for a gate check) is the way to clear up some space in the cabin. I don't see that happening though as Bag fees are a huge cash cow