Haven't read any of the replies, but I can tell you I would and have taken the carseat on the plane when my kids are around that age. Even when we didn't need it at our destination. And, honestly, thank goodness I did. When we went to WDW in 1996, DS was about 16 months and he got so sick! He was breathing terribly. We were fortunate in that Disney CM came and took us in a hotel stationwagon to the urgent care and then to the pharmacy! Had we not brought his seat, I'd have been a nervous wreck, and CM told us that they wouldn't have allowed him to drive us around without it. Another time we were flying and had to be grounded in another city due to problems with weather at our destination. They took us to a local hotel. Happily, I had the car seat and didn't have to worry.
However, the number one reason that I take the car seat is a recognized and comfortable/familiar way to keep my children safe and seated during a flight. Especially if it's going to be a longer flight and especially if it's going to be during a time when they are sleeping. That five point harness keeps them alot more comfortable and safe.
Oh, and as far as the lugging it through the airport thing, we have car seat carrier that we bought when DS1 was a baby. Fit car seat, blankie/lovie and even sweatshirts, lightweight jackets. It had a handle and a shoulder strap. However, we still just put it on a luggage roller (I think they're called luggage racks, but I don't remember) and take turns pulling it. Not a problem.