For shorter rides in the city (I live in the city), no cab driver EVER refused to take me and my kids because I did not have a car seat. (I have a five year old and a 10 month old). I know it's illegal, but there was no way I was going to lug a car seat to and from day care to our home. (Sitter would bring our then 15 month old home after day care). I got the taxi idea / no car seat from other parents who sent their kids to our daycare.
Aside from safety, I like bringing a car seat to the airport even if it is bulky because it's cheaper than renting one later. I always check in my car seat with the rest of the luggage as I have enough things to carry to the gate. Besides, I read that my particular brand of carseat is too big to fit on an airplane seat.
Have fun on your trip!