I personally think they are incredibly dangerous to motorcyclists and cyclists. When a car with spinner hubcaps pulls up to an intersection it takes much longer for other vehicles to tell that it has actually stopped (because one of the main visual cues normally is to look at whether the wheels are moving) so a couple of times I have found myself on the verge of taking emergency evasive manoeuvres thinking the car is pulling straight out on me (cars pull out on bikers all the time). I've seen cyclists swerve dangerously also thinking their lives were in danger from a car with spinny hubcaps.