Everyone who realized they didn't need their big rollaboard in the cabin (along with their tote) will bring it on, one that they would have otherwise checked. All that will do is delay flights while frantically trying find space, then to check bags last minute at the gate when the 150-200 suitcases are wheeled on and don't fit.
There is a stockholder's meeting today with AA, so they may be trying to look good for that by making this announcement on the same day. There's a good chance it may not stick since it's just a testing period right now and coupled with their announcement of reducing 12% of their schedule in the 4th quarter, and parking about 100 aircraft due to oil prices and layoffs by then, they have to come up with something. Considering the low fares due to deregulation and cropping up of all these startups who charge practically nothing then don't make it, the majors who had some "frills" make more money on air cargo and mail than they do from passenger tickets (although they still need them) so the hit is the cargo hold.
I do agree they should just add $$ to the ticket since fares change all the time, but the first point of contact of a customer is comparing prices from here to there. Unless every airline does the same thing, they'll just go with the "cheaper" alternative so that area has to be competitive. Any taxes or airport fees are not pocketed by the airline, btw so that can't cushion their costs. It is a really bad time right now so there'll be tests on what to charge for and what not that was free before oil prices went from 30.00 to 130.00 a barrel. An airline tested charging for sodas, but that didn't pass and I know some charge for pillows and blankets. I never thought I'd see the day we would have to pay for food on the plane or some don't even have food at all, but it's normal now.