I know here in NJ, if you don't have your quad, dirt bike, or whatever you have, insured and registered, if you get stopped by the police than you get it impounded because you can't prove you own it, and it's not a legal (off road/all terrain) motor vehicle in NJ. And you can't get it out of the impound lot with out the registration and insurance. I only have mine done so I'm not hassled by them.
I'm not sure if it is meant to deter theft, as much as it is to get a few more $$$ from us that own them. But if one does get impounded they check the numbers and see if it is registered to anyone, then they contact the person to have them come retrieve it. BUT I am not sure if mine was stolen, then impounded, if when I go to get it from the impound if I personally will have to pay the charges of the tow and the spot in the lot ($85 just for the hook up for tow, then however much in the lot per day cost).
But since I don't know if registration would deter theft, I didn't vote in the poll.
~Bill