Silencers are illegal in every state I know of unless they have been registered and the tax paid. The form 4 is an affirmative defense against prosecution (and unnecessary hassles by the cops). There is no law saying you must keep the form with you. There are however people whose job it is to arrest people who are breaking the law, this brings us back to having an affirmative defense.
Stopping power: The power some guns have to make you stop what you are doing and get your wallet out.