For me, the nice thing about have a car seat with a base (I have the britax belted base as I don't have isofix) is that when you are going out in the car and you're tired/in a rush/its raining etc etc, you just click the seat into a base that you know is installed properly (because, like me, you will spend about a day before baby is born getting it perfect!). I like knowing that once the seat is clicked in and the green button pops out to show is is in properly, I know its all good to go and there is less chance of me making a mistake.
Also if you are just going to the shops quickly or to do the food shopping it is a lot easier to just use the car seat, either on the trolley or on your travel system base because you won't be long and for me it means more space in the boot for my shopping if I'm not carrying the pushchair or pram.
J x
