The question is: at what cost and how much is the benefit, really? Each parent has to answer this personally for him-or-herself. For me it is definitely not worth the extra cost.
For my family, the cost of one extra airline ticket compared to the total cost of the vacation is minimal. If I had to, I'd knock a night or two off of our trip to be able to afford the extra ticket.
I understand that not everyone can afford to buy their baby a ticket, but I think that most could find a way to do it if they tried. When you add up what the average family spends per day at WDW including resort cost, park tickets, and food, then knocking a day or two off of their trip could easily cover the cost of an airplane ticket.
As far as the benefit, well for me it is knowing that my baby will have the same chance at survival as me, my husband, and my older child if something were to happen. What if my plane is the one you hear about that skids off of the runway after landing? What if my plane is the plane that happens to have the severe turbulence that throws an unbelted passenger to the roof of the plane?
While rare, these things do happen every year. Don't you think the people on those planes that it happened to thought the same thing as most of us....that it would never happen to them?
Yes, air travel is safe. Much safer statistically than traveling by car. But humans do make mistakes, machineray fails, and nature is unpredictable. So things can and will happen.
By the way, my father is a former pilot and is in 100% agreement with me on this topic. He has experienced firsthand the kind of turbulence and events that make seatbelts on a plane neccessary, and he along with many, many people in the industry have very strong opininions that it should be mandatory for every child to have a seat and be secruly strapped in.
And I trust my Dad's opinion 100%.