The problem with NICS is that we convinced ourselves of the need for it. Is there a drop in violent crime that can be associated with it? Nope.
Tom, I think we really need to be careful in presenting this argument as we will fall into the trap of making a case for registration (ie: we need better visibility in order gauge the effectiveness of our controls).
To my mind, this subject is our achilles heal. If we don't successfully argue it, either pro with limited scope or against, it will be expanded into a significant threat (registration).