I have a 4 1/2 year old that only weighs 36 pounds and is still in a harnessed car seat. I am bringing it with me. She is used to using it on the plane and I can't imagine having her in a seat on the plane without her seat. They look so small in the regular seat belts on the plane.
Some of the harnessed boosters are very lightweight and very easy to travel with. Plus, I have heard horror stories of getting to a car rental place and the seats there are dirty, old, or just plain terrible. I would never trust a rental place for a carseat.
We also use the seat as a 'stroller' through the airport for her. I have some unattached wheels and I bungee the seat to the wheels and she sits in the seat 'indian' style and we just pull her along. Works great for us.
I won't book any excursions that are make me leary of carseats in a taxi. I can't imaging taking a taxi in Cozumel without a carseat! Yikes, makes me squeemish just thinking about it.