As both a person who has spent many years advocating for increased use of car seats/boosters in automobiles and planes, as well as a very frequent flyer, I'd like to thank all of you who plan to use your children's seats on the airplane.
As Gail mentioned before, turbulence is the enemy here, as children become virtual missiles when it gets very bumpy. I have seen a year old child literally ripped out of his mom's arms on a particularly rocky flt. She was in tears (and so was I)- she honestly thought she'd be able to hold on him! Luckily, no one was hurt, but everyone was scared!
The seatbelts on planes are not meant for small children and the forces that move the plane in turbulence are so strong that they can cause seatbelt related injuries- just the same problem we had with lapbelts in cars, and why they are no longer recommended for kids.
You will absolutely not regret using the carseat on the plane. My DD was actually sad when we switched to the high-backed booster w.o straps that could not be used in the plane (she was too tall for the straps, but too light to ride w.o the booster in the car). She said she missed her "snuggly" seat!
ps: She rode in her booster in the car until she was darned near 6 years old, 50+ lbs and about 4'3" tall. I now know based on new studies that I should have kept her in that booster longer...luckily we drove a car with a seat belt that hit her in the right spots, and of course nothing happened to her, but I regret having taken the risk.