First, I agree with you, but maybe say it a bit nicer? We 'try" to be civil and nice to each other on the forum, although I am not always that way myself
This is thought of by many people as keeping the numbers secret a "security" measure, for the following reasons:
1. Some one will see a luger and claim that it was theirs years ago, by hiding the serial number, no one can claim this. I have never heard of this actually happening, although I am sure it has; although I am inclined to believe that it is more of an urban myth. That said, I sold a luger to a gentleman in Calf and when he went to register it, it came up as a hit of a stolen weapon. The new owner contacted me, and I contacted the officer in charge of the investigation, indicating that the sn by itself had to be combined with the year, caliber, toggle markings and if there is a suffix, the suffix. The gun was allowed to be registered...
2. Folks worry about other people knowing the sn of their guns. Unsure why
3. They have seen other collectors do it for years, so they do it.
Ed