All of the vehicle sensor data could be sent to a laptop. This would allow:
vehicle maintenance to be tracked precisely

useage profiles generated for each driver

utilities for providing drivers with cues related to fuel savings

automated vehicle useage information for tax purposes

automatically tracking vehicle costs related to individual customers

Allowing drivers to download extensive and selectable playlists into their vehicles audio system.

Providing manufacturers with the avenue for new products like: designer external lighting, flexible data displays (advertising in the back or on doors), free form decorative LCD panels for hoods and trunk lids, adjustable intensity tinting for windows, directable cameras, headlights that track steering movements, security systems which include the towed trailer, security system for strapped on bicycles and adjacent motorcycles, child monitoring systems, vehicle to vehicle communications, sharing info between vehicles, collision avoidance systems, garage door openers, automatically setting home security, documenting vehicle parameters for improper police stops, providing a local wi-fi-like connection for all the passengers, allowing for a large hard drive to be accessed in the car (nav data, music, downloaded websites, peer to peer data), ...

This allows for the cell phone with bluetooth to distribute the internet connection to all of the passengers. A Bluetooth to Wi-Fi adapter could distribute the connection even further. Multiple Bluetooth headphones could each be tuned to different music archives. Passengers could email or text message collaboratively.

Cell phones that are bluetooth compatible download functions for monitoring and controlling everthing related to your vehicle. In addition it provides on-board systems with access to the internet. Online mechanics could be contacted to diagnose problems with your vehicle.

This would also breach people's privacy. Many people don't like giving up their privacy. The government would likely provide themselves with a way to read this feature themselves to track your vehicle and the vehicles locations and whatnot. Many people would not go for this.

Adding all that control via Bluetooth opens the door for hacking and security issues with your car and it's systems from mechanics trying to screw your car up to do more work on it to companies hacking your car's GPS travels etc.

Adding a 128-bit password phrase is easy enough to include. The computer you are using has this capability. That's why credit card transactions are secure as long as you ensure the company has an https on it's payment URL. Security isn't much of an issue; it's easily implemented.