Heres the thing, having a title and stuff for your vehicle really does help you in the long run. It is easier to sell when you want to and when it does get stolen it is actually in the system with the DMV. If you dont have papers for your quad when it does get stolen do you know the vin number so when you file the report you have one. Because if you dont your never going to get your bike back. Why because your vin is not in the system so they wont know that it is even your bike when they stop the guy who stole it riding down the road.
Second, you were not riding it when he saw you but it is pretty common since that you rode it there to be stopped on the side of the road changing your plug. Same reason you can get a DUI when your drunk, drive and then decide to sleep it off on the side of the road. Cop comes, he can obiously tell that you drove the car there drunk and can give you a DUI.
Prety much everything that a cop does in the long run will really help you out. Hes not taking your quad to be a dick, he is taking to to pursuade you to get the proper papers for it.
And once again I will use this argument: Were you doing doing something you were not supposed to be doing when you got caught? If the answer is Yes, dont complain.