We didn't take the carseat for my very petite 2.5 year old daughter when we went in May. Everything was great. She was as comfy as if she was on our sofa. My wife, a Fellow of the American Academy of Pediatrics, knows the AAP guidelines but it was her choice not to take the carseat, since the MINIMAL safety enhancement didn't outweigh the HUGE pain-in-the-butt of lugging around a car seat that we weren't going to use except on the aircraft. The way she explained it to me, in a car you need the 5-point harness to protect against the forces most likely to occur in a car crash or sudden stop. In an airplane, a lap belt is sufficient to protect child or adult from the most likely problems/forces.
The bottom line is, no matter what we all say, do whatever makes you the most comfortable. If you'll be nervous the whole time on the airplane WITHOUT the carseat, then it'll be well worth it to you to lug that carseat around wth you -- you'll be lugging it for maybe 20 minutes total round-trip, versus worrying for several hours during both flights.