Flexibility. On some cars you can only mount the rig on the roof (and visa versa), whereas sometimes the bonnet can be used, so obviously if you're shooting from the roof you need an extra metre or two to get the same kind of shot.
It depends entirely on what look I'm going for. Also, some locations (such as wide open countryside) you'd want the car to take up a little less of the frame and let the scenery make the image. Other cases (such as underground carparks with lots of lights) you tend to want the car to almost fill the frame.
At the moment I'm shooting on a crop body with an 11-16mm so I need all the length I can get, when I step up to full frame I will be able to get away with a shorter rig. With new house settling next week though, a FF body is a long way away
