Bernstein may be right. He certainly does care about this issue, and has researched it. But when there are two almost equivalent approaches, one has 0 risk, and the other has minimal, why not follow the approach with 0 risk?
Incidentally your Berne Convention argument is rather weak according to my reading of the actual text of the Berne Convention. (See http://www.wipo.int/treaties/en/ip/berne/trtdocs_wo001.html for the text.) Public domain is only referred to in article 18, and the only type of public domain referred to is due to expiration of the term for copyright. Section 7 indicates the minimum terms in question, and those terms are both long and do not contain anything indicating that they can be shortened by the author's wish.
Therefore the fact that public domain is mentioned in the Berne Convention does nothing to reassure me that countries which sign the Berne Convention will necessarily pay any attention to a public domain declaration.
They recognize that the public domain might not exist in all legal domains, so a licensed version is available, for a fee.
Others have decided to decline to use copyright protection. There's a list of such software at http://unlicense.org/ .
I wouldn't look to the Berne treaty for some statement of the international existence of copyright law. You need to look towards national laws instead. For example, the US recognizes the public domain, and a work of the United States government is automatically in the public domain in the US. (Though it might not be in the public domain elsewhere.)
So like any social movement, if enough people release software and disclaim copyright protection, then those jurisdictions which don't recognize the public domain might change. If no software ever takes the risk, then it will never change.
Incidentally your Berne Convention argument is rather weak according to my reading of the actual text of the Berne Convention. (See http://www.wipo.int/treaties/en/ip/berne/trtdocs_wo001.html for the text.) Public domain is only referred to in article 18, and the only type of public domain referred to is due to expiration of the term for copyright. Section 7 indicates the minimum terms in question, and those terms are both long and do not contain anything indicating that they can be shortened by the author's wish.
Therefore the fact that public domain is mentioned in the Berne Convention does nothing to reassure me that countries which sign the Berne Convention will necessarily pay any attention to a public domain declaration.