We always use carseats on the plane. I feel very strongly that seats can get damaged by baggage handling and that could severly limit the safety of the seat - but you would not know about the damage.
I always paid full price for my "lap babies" to have a seat so we could use the carseat for safety. IMO it is not an inconvenience. I'm a single mom and if I can "deal" with a carseat in an airport and on a plane, I think most two parent family should be able to work it out. I wouldn't take a trip without my camera, and I won't take a trip without a carseat either. You just need to figure out what works for you. I've used the luggage cart in the past and now I just carry the seat, simplicity works for me, LOL! I only take a large purse for carryon, and gate check my stroller.
Aside from safety, IMHO it is more convenient to have a car seat on the plane. My kids are used to traveling in the car. They know when they are in their car seats they are expected to stay in them and behave. They don't ask to get up and move around in the plane because they don't know that is an option. The carseats are more comfortable for them, they can sleep in them where I don't think they would be comfortable sleeping without them.
I currently travel with Britax Roundabout. This seat is similar to the Marathon but has a smaller frame that is easier to get down the aisles of the plane than the Marathon. It also has a handy bar on the back to carry it, making it much easier to carry than the Marathon and also weighs about 5# less than the Marathon. I estimate that my daughter will outgrow the Roundabout by height before she turns 4, so I wouldn't recommend buying one for an older child because they are quickly outgrown, but it worked great for us because we bought the seat when she was a baby. When she outgrows the Roundabout I'll probably buy a Radian, which is a really heavy seat to drag through the airport, but it folds so at least it is easy to get down the aisle of the plane.
