Don't car kits all use Bluetooth now anyway?
Sure, you may still need a connector for power to the phone but given Bluetooth is a standard, it would mean that you could use any BT phone without the need for special cables which only one set of devices from one manufacturer can use i.e. all BT phones would work.
Now people will go on about the stereo integration - Would it not be currently possible to have the phone streaming music over BT to the car unit which will cut out when making a phonecall/getting satnav directions etc? I know it does this for the 3.5mm aux out already.
Also, Bluetooth controls should allow you to control the device for skipping tracks, volume etc....
This may sound like another swipe at Apple but given that every other manufacturer is using the micro-usb standard and FULL BT compatibility, it would be more prudent for Apple to change.... IMO