As a former flight attendant, I can tell you that children REALLY should be in restraint devices on the plane. A mother cannot hold on to her child in extreme turbulence or a crash landing. And, while people in restraint systems have a "chance" of surviving...unrestrained babies do not.
Here's some things to keep in mind:
1) 95% of all accidents and extreme turbulence occur during takeoff, landing, and descent. So, the devices that can be used only in flight will protect your baby for statistically only 5% of the danger time. I don't think they are worth it.
2) Car seats that face forward, and do not extend past the seat cushion can go in the middle or window seats. Car seats that go backwards must go in the window. No car seat can go in the aisle, as a child could be seriously injured if a bin opened and luggage landed on the child.
3) I began working as a f/a in 1994. It was never part of my company's emergency plan to have babies sitting on the floor.
4) The FAA knows it is in babies' best interests to be in restraining devices, but the FAA is charged with a dual mandate...promoting air travel and ensuring air safety. Studies show that families would begin driving if they had to pay for their babies. That would damage the already struggling airline industry, so they decided to continue to allow babies to go unrestrained. They feel babies are statitically safer in a plane than they are in a car.
Every parent has to decide this issue for themselves. However, I never met a single pilot or flight attendant who would allow their children to fly as lap children. For us, it was too risky. Granted, we flew for free...but, we flew behind standby, and often would have to wait for another flight or make our child a lap child....everyone I knew always waited for another flight.
Statistically, you are VERY safe on an airplane. You just have to decide about your comfort level with the "what if". But, whatever you choose...don't fret about it. Just make the best choice for you and your family, and try to relax. Flying can be stressful enough.