Industry standard is 50lbs per checked bag, one free "bin" bag and one free personal item per paying passenger.
I suspect Spirit knows at least some of their customers won't do their homework.
JMO but I think the disclosure of any non-standard policies should be made in large print on the screen where fares are displayed before a potential customer even starts to reserve a flight.
Spirit has a list of "optional fees" one of which is a mandatory "unintended consequences fee".
I suspect other airlines won't start charging for "bin bags" but might start enforcing size requirement. Might establish, and enforce, weight restrictions.
The thing is, some people do do their homework and would choose Spirit even with all their bag fees. I went in understanding that they charge for both carry ons that go in the overhead and checked bags, that their weight limit for checked bags is 40lbs, and that they don't serve snacks. I go to WDW several times a year, so am pretty much always looking for airfare. I check airfare prices pretty much daily.

When I found airfare for both my May and Sept trips on Spirit for $148.00 (plus bag fees and seat fees), I jumped on it. Even with all the fees, no other airline flying out of DFW or Dallas Lovefield was even close. $148.00 (plus fees) vs >$450.00 was a no brainer to me. I am a light packer so, would never go over 40lbs for a checked bag anyway, but can easily pack in a carry on if I want/need to, have no problem paying for the seat I want (always bought the $15.00 or $13.00 seats on Air Tran anyway), and can always bring my own snacks. Again I couldn't care less about the extra fees, as long as the final price is less than the other airlines. Southwest offers two free checked bags and carry ons are free. However, their prices out of Dallas are
alot higher and they don't have nonstop flights. Why pay over $400.00 for a flight on Southwest with a layover when I can pay $148.00 (plus bag fees of course) and get a nonstop flight on Spirit? just to say I could have carried two checked bags for free if I had wanted to? or that I could have packed 50 lbs of clothes if I had wanted to? or that I didn't have to pay for a carry on? Sorry, I look at the final price after all the fees and go with the cheapest, and even with all their fees Spirit was about half what all the other airlines were. Thanks, I'll pay the bag and seat fees and still pay less for my ticket. As long as they get me from Dallas to Orlando safely, I am not a picky flyer. I hate to fly and usually just try to go to sleep ASAP anyway, so really couldn't care less about free snacks either.
I really wish airlines would enforce size and weight requirements (especially for people coming home from WDW). I sometimes can't believe the huge bags people try to bring onto the planes as carry ons anymore.

Coming back from WDW how many times do you hear the announcements that the plane is full and all that will be allowed on is one personal item and one carry on that fits wheels first in the bins. Yet you see people carrying on carry ons that are bigger than my checked bag, personal items that are at least 24", plus a couple of huge WDW shoppings bags and then they looked shocked if the gate agent says something to them.

It's like they expect everyone else to follow the rules, but the rules of course couldn't possibly apply to them.
