There's really no reason this can't happen right now.
Take a look at the shared data plans currently being heavily promoted by both AT&T and Verizon pooled data plans that allow you to have up to 10 devices share the same pool of gigabytes of data; there's really no reason that a data pipe directly into your car couldn't simply be another device.
From the carrier perspective the value is an additional monthly access fee for the device and the consumer likely jumping into a higher data tier to accommodate the increased data consumption
On the consumer side, it makes it even easier to consume data and data-intensive services in the car without needing to pair your phone or even remember your phone in order to make use of them (note that if you're like me and you never remember to charge your phone in the car, it also means that you don't have to run all the battery-draining services to power the cool stuff in your car).
Also on the consumer side is a more seamless experience, especially for the non-prosumer: just turn on the car and fire up Pandora or let everyone in your SUV browse the internet, as opposed to having to pair with Bluetooth and turn on the hotspot mode on your phone.
This isn't the same argument of "I already have a cell phone, why do I need a car phone?", this is fundamentally more different based on how the carriers are now pricing data and devices and is due to the level of connectivity we expect in our vehicles.