Adding my 2 cents worth here. I've done both. Lately, I've only been comfortable flying with the carseat for my now 15 month old. My main reason for using the carseat is turbulence or problematic landings/take offs. In these cases, if an accident were to happen, it could potentially be survivable, but it would decrease if the baby were in your lap. Chances are, you won't need it, but I'm super cautious, so I can't function with the idea that my child isn't in the carseat (literally, I obsess until I buy the silly seat and then curse the carseat while I'm tripping over it in the airport, then I repeat all over for our next flight). It is really another of those fun parental decisions that only you can make. Will you need the seat? Probably not. What if you do need the seat? The thing that pushed me over the edge is that if you are having an emergency landing, you will need to stow your BABY under the seat like a piece of luggage. Yuck! They are simply not safe in your arms. With crash forces, they are likely to be ripped out of your arms. If they are not, they will become your airbag (you'll be pushed forward and your baby becomes the airbag). I can not live with either of these scenerios, so I buy the seat and drag the dreaded carseat with me. Yes, it is a pain. I seriously considered the Baby B'air, but you can't use it during taxi and landing. So, you still have nothing for the times when you need it most (a survivable crash would likely occur during these times). It would definitely help during turbulence, but it doesn't solve that pesky airbag issue (they are still your airbag since they are in front of you).
Basically, no one will know if you'll need the carseat. You probably won't, so it is probably fine not to bring it. But, you might, and unless someone has a functioning crystal ball, all we can do is tell you what we did and whether or not it was necessary. Afterall, personally I've never needed mine in my car, but I still consistently use it for every trip.
On the plus side for bringing a carseat is that both of my children have spent almost all of their flight time asleep in the carseat. I was afraid to bring mine, thinking that I would have an empty carseat next to me taking up space. But since our kids are used to the carseat in the car, they seem to sit quite happily in it (especially with milk, food, crayons, etc). Usually mine have been asleep within 30 minutes and sleep for most of the flight! Yeah! I guarantee that mine would not have slept so well on my lap (especially my little one, he never sleeps in my lap -- just more fun to explore and jump around).
Sorry this has turned so long. This seems to be a heated subject that has no true answer. I completely understand why my friends and family do not fly with a carseat, it is just not the same choice that I make.
Have a wonderful trip,
Jennifer