Then you have to lug the car seats round with you wherever you are.
Say I want to go to London for a weekend. It's a bit too far for the kids to walk to the local station (especially if it's raining), so we drive to the mainline station, leave the car, and the car seats, in the carpark.
With a taxi I'd have to
1) Hope the seats fit in the taxi
2) Take them on the train (store them where?)
3) lug them on the tube (including a change at Green Park or Bank or somewhere)
4) Go straight to the hotel and check in
