I'm happy to hear you can even turn it off while moving, to be honest. I figured it would be a "must be in park and push these seven buttons for ten seconds" kind of jumping through hoops.
My car only has the speed limit graphic changing color (well, it can also beep but I turned that off before I even put the car in drive for the first time). I know it has an adjustable margin, but if I remember right it only has two settings and neither are high enough to compensate for the inaccuracy in the speedometer at highway speeds. Not to mention that I actually want to speed a bit under a lot of circumstances, so ideally I'd want it to have separate offsets for each speed limit.
Annoyingly, the car actually knows the exact speed even though the speedometer always shows higher than real speed. If I zero out the average speed measurement and maintain an exact speed, that will show the correct speed. But, for some reason they've decided the speedometer should show a few percent more than actual speed, giving a higher inaccuracy the faster you drive.
As for the system causing more risk than it solves, what matters is how it pans out on average. The statistics over the next few years should show whether it has any measurable effect whatsoever. My hunch for the speed thing is, not really. I don't get the impression that people unintentionally speeding is a cause of many accidents. I somehow doubt the people who prefer to turn it off will start running people over while doing so either.