Colin
Most of your answers are in the comments at the end of the thread. It is in the interests of the supply companies for you to have one, that's why they are constantly pushing to have one fitted. There is little or no benefit for the consumer and it has been suggested that eventually the supply companies will have control over what you can use and when you can use it. (I'm not so sure about this but who knows?)
There is also the question of compatibility if you change suppliers. Check out the various forums discussing the pros and cons of having one