Well, or to supplement a flight with a rail leg. There are plenty of cases where a flight to one of the relevant airports (MCO, WPB, FLL, MIA) will be far cheaper than one's ultimate destination, and especially when MCO is the cheap one (MIA and FLL have "expensive" reputations from what I recall while MCO has a "cheap" one) there will be plenty of cases where AAF can "steal" a passenger bound for South Florida from the airports there and get them to fly into MCO instead.
Depending on AAF's business model.
If they can convince some airlines to codeshare with them, this would indeed be an attarctive proposition. If on the other hand the airlines chose to close the door on negotiations, I doubt many travellers would go to the lengths of booking the train journey as a separate leg, especially if the booking restricts them to a given train and a delayed incoming flight might invalidate their train booking.
This relates to Orlando to Miami traffic of course. If you're looking at traffic from Orlando to intermediate points the story is different.