I had a subscription to OnStar on my vehicle and while using it I enjoyed it and realized its practicality. The main benefits of OnStar for your car are:
Automatic Crash response
Stolen vehicle assistance
You can also make hands free phone calls, and if you lose the location of your vehicle in a huge parking lot, for instance, OnStar will help you find it by beeping your horn and/or flashing your lights. I used OnStar in lieu of a GPS and appreciated being able to have a human being providing me with directions so that I could get from place-to-place. However, eventually the cost of the subscription just didn't seem to be cost efficient for me and since I was unable to get them to lower their price to keep me interested I went ahead and got my own GPS.
Now I realize that while I had OnStar that in all likelihood they could track me down to my exact location to the accuracy of their device which I suspect is around 30-40 feet of your true location and further that they could probably extrapolate my speed and basically reconstruct everyplace that I went with that vehicle. For a criminal, this would be a big problem, for me it really wasn't a big deal, but it was something that I wanted to be cognizant of. Not too surprisingly, they later came out with a "family link" in which OnStar is so kind as to provide you, the subscriber, with the exact location through your computer of any vehicle signed up with the plan. That is a definite game changer.
The difference between me signing up for a service is if I am at all diligent, I know the good and the bad, of what the service can or can't do for me. If the bad outweighs the good, I can simply walk away and be done with it. Now when another driver, like perhaps your spouse, your significant other, or your child, gets into the car and drives, they may be completely clueless about the score and therein lays a serious problem. Yes, sure, it's probably meant for good, but in this case there are massive unintended consequences.
Specifically, if you know where someone is at and they don't know that you know, you are in a godlike position and that gives you an incredible power over them. It puts you in a 'sting-like' situation in which you can just gather information over a period of time before deciding how best to use it or if you feel an urgency you can react to this information in virtual real-time. Quite frankly, although the system is ostensibly setup to make sure that your loved ones, for instance, arrive at their destination safely; I believe that the family link is really setup for you to find out whether your loved ones are going to a destination that isn't on the approved list.
It's the nanny state, except run by yourself with no supervision, other than your own conscience, and I suppose your own justice system.