You are setting up a staw man. So unless eternal perfection is achieved it is not worth trying? Laws against murder have yet to stop murder completely. Should we give up on that too?
The difference of course being that murder violates the rights of another human being, whereas drug use in and of itself does not.
Government exists to defend rights. If a drug user violates rights, then by all means hold him accountable, and punish him harshly. Otherwise, leave him the hell alone.
It is your kind of argument (that government should attempt to prohibit inanimate objects which are not criminal in themselves, but might be associated with crimes) that is used by anti-gun activists, anti-alcohol activists, etc.
It's the road to tyranny.
Give me one good reason why beating someone's head in while stoned on Methamphetamine is more heinous a crime than beating their head in while "in a pissy mood".