Yup-a friend of mine is a paramedic (and another is an EMT training to be a fireman) and she says it's easier to stabilize/transport children if they're still in their car seats. If, per chance, they have a back or neck injury, the car seat will keep it more or less stabilized until the doctors can treat the child.
Also, the car seat can be strapped down in the ambulance for safer transport and to make room for another patient (if the child is stable).
One last thing is if you ever have to intervene after a crash but before the police/EMS arrive (i.e. the car's on fire) cut the seatbelt/LATCH strap of the car seat if you can and remove it with the child for the reasons explained above.