tttessa,
I don't really see your reasoning.
The luggage is packed into containers on the aircraft, not strewn aboutb the cargo hold.
car seats, soft luggage(bags) are all kept seperate from suitcases when loaded into the containers.
The temperatures within that hold are within the operating parameters of the car seat.
they are all regardless of which country they are used in, produced at the same factory or to the same material make up, at least with your own, you know the histroy of the car seat, the use its had, the weight of persons using it/playing around in fun in the car etc.. something you have no understanding of with rental ones.
The major problem with car seats is the great stress they are put under when subjected to the energy absorbtion at the time of a car crash, going from 60mph to 0 in seconds, puts strain on the attachment areas, not the small bumps you will get in day to day use.
Secondly, why pay for a seat which may not suit your child, unlike their own one, easier for them to adjust to especially after a hard flight/day.