I've also heard the argument that having a national registry makes the possibility of future mass confiscation much more palatable.
There are probably a good deal of people who wouldn't mind amending the Constitution to rid it of the Second Amendment but who still view the 4th Amendment favorably. It would be very difficult to enforce a policy of mass confiscation without the aid of a national registry that wouldn't be a violation of the Fourth Amendment.
All 20th Century gun confiscations I can think of, aside from the Chinese Communists' rifle tax system, were preceded by registration. Nazi Germany most infamously, and in occupied territories like the Netherlands failure to produce yours would result in your entire family being summarily shot in the village square. But also Cuba after Castro took power, or less dire (so far), the U.K. bit by bit. Or closer to home, California, after the AG decided that SKSes were banned after all.
Last time I checked, which was decades ago, the national ACLU, which has no respect for the 2nd Amendment at all, did have such a 4th Amendment objection.
There are probably a good deal of people who wouldn't mind amending the Constitution to rid it of the Second Amendment but who still view the 4th Amendment favorably. It would be very difficult to enforce a policy of mass confiscation without the aid of a national registry that wouldn't be a violation of the Fourth Amendment.