I think there's a key distinction here. Personally, I understand that it's no real secret to the government or to many people who know me that I have some guns. FFL records, GWCL records, and the like all make that pretty clear. Even if I had only bought guns from private sellers, I still need the GWCL to keep myself safe and still stay on the right side of the law.
If laws changed and the government hoovered up all that info, they would have some ideas about certain guns that I once purchased, but may not own now. If they wanted to tax, confiscate, or otherwise interfere with my ownership of those guns, they would have a good place to start but their list would be quite incomplete. If everybody I deal with has records of who they sold to, the government could grab that info as well and get a much clearer idea of what guns I probably do own. That makes me more uncomfortable. YMMV.
