We guess it had to happen sooner or later, and indeed some Airfarewatchdog readers have been advocating this. Spirit Airlines today announced that it would charge up to $45 for carryon bags, which is even more than the airline charges for checked luggage.

The new fees are effective for fares bought on or after April 6 2010 for travel August 1 and beyond.

Is this purely a revenue move? An argument can be made that if indeed airlines are justified for being compensated for the extra fuel and labor that passengers' luggage costs them, then doesn't a heavy carryon bag also consume fuel? And more to the point, if no one brought carryon bags into a plane, wouldn't that plane spend less time on the ground, allowing the airline to utilize its aircraft more efficiently?

We have no idea what's really behind Spirit's move, but we do wonder what other shoes will drop. Fees for credit card and debit card use? Fees for in-person check in at the airport, a move that Europe's Ryanair has already adopted?

And will other airlines follow Spirit?

Spirit's new fees range from $45 for carryon bag fees paid at the gate, to $30 if paid online or by phone, or at check in. Members of Spirit's $9 Fare Club pay $20. Exempt are small personal items measuring no more than 16 by 14 by 12 inches that must fit under the seat, not in the overhead bins. And carryon items may not exceed 22 by 18 by 10 inches. These are higher than Spirit's checked bag fees, which start at $19 for the first bag and $25 for the second if paid online.

This could save airlines millions

So it seems clear that the airline is hoping to discourage carryons altogether.

After all, how much faster could an aircraft board and deplane if no one brought on luggage? I'd guess 30 minutes at least, total, on both ends of the flight. Thirty minutes times how many flights per day in the U.S. commercial airline industry? Do the math. They could save millions by buying fewer planes if each plane could do more turnarounds each day and spend less fuel keeping the AC running while passengers cram their steamer trunks into the overheads.

We predict that more people will be using FedEx Ground and UPS Ground to handle their luggage. Maybe airlines should link to shipping companies, encourage passengers to use them, and take a commission from the fees.

After all,  airlines really don't want to be in the luggage hauling business. They could make a lot more by using their cargo holds hauling freight and mail, so quite frankly they'd be just as happy if you used UPS and FedEx,  although they have become somewhat addicted to those bag fees in the same way governments have become reliant on sin taxes.

Better for passengers, too?

A few years ago, my friend Mike was sued by a passenger on BA when his carryon bag fell from the overhead and hit the guy on the head. And how many people have thrown their backs out hoisting heavy bags into the overheads? How many fights break out each day over scarce space in the bins? Maybe we should dial back the clock to the grand old Mad Men days of air travel when people just carried on board one of those airline-logo vinyl bags, a coat and a hat. Well, maybe not a hat.