Onstar's unwanted feature is the remote controlled microphone. Look around you, how many asshole chavs are in your office? That same percentage of asshole chavs are in the police, your banks bank office, an OnStars controls room, at your ISP, your mobile phone company. So it's better to remember the chav factor when buying technology that can be abused against you.
Really, can it work any better than an immobiliser? If I added an immobiliser that turned off the power after a while if the key wasn't present, wouldn't that be the same? Officer chases car, but not so hard, at some point the stolen car turns off, officer arrests perp without need for kill switch.
Also suppose you're speeding, a little over the limit, but not worth chasing you and issuing a ticket.... no need, just turn off the car, turn on the microphone and speaker and officer issues a verbal rebuke, or a little 10 minute timeout. Taser for your Transam!
That would be great, cark park companies could add terms to their car parks. Instead of clamping cars, "badly parked cars will be immobilised by OnStar". Pay a little fee, OnStar will turn off your car saving the car park from clamping!