"You ought to be paid to sit in the middle seat, let alone pay to sit there," Mann said
Mann said the ideal model for passengers is Air Canada, where passengers get $5 back if they don't want to check a bag.
By charging this new fee Spirit can lower its base fares slightly -- not necessarily enough to supplant the new fee -- which means the airlines flights will show up with lower fares on travel search engines, Mann said.
"It's become very, very confusing and basically games the fare search engines out there," he said. "Passengers don't necessarily think logically. They just tend to pick the lowest fare."
Travelocity's Brown said some passengers "might be a little bit more tolerant of something like this" if the airline is offering "rock bottom fares," adding that this could make the boarding process more pleasant. Currently, she said, the No. 1 complaint people have about boarding is other passengers whose bags are too large to fit in the overhead bins.
George Hobica, president of airfarewatchdog.com, also said this will help planes load faster and wondered how far airlines will take this process.
"Who will be the first to eliminate airport check-in counter staff?" he asked, "Forcing passengers to show up at the airport with a pre-printed boarding pass and pre-paid luggage fees, then proceed to a conveyor belt, show their fee payment, and drop their bags themselves on a conveyor belt - and proceed to the gate. No human interaction necessary."