As a person that worked on a traction control system, a driver is not going to notice the difference between 5% slip and 15% slip, but 5% slip is much faster, (and faster than 0% slip) A light that went off at 1% slip would be an enormous advantage. It wouldn't be as good as a traction control system that is making 1000 changes per second, but it would be huge compared to nothing at all.