John pointed out that, in his scenario, one could lock, create with an empty body, set ACL, then write the real content. A compromise suggested on the list was to define the ldap-URL property, which points to the principal's LDAP entry. Different people will care differently about LDAP integration. Larry says the first one strongly depends on context, on how the spec gets written. Eric states that he does not want to make the DAV server replicate data from the LDAP server.
Nobody can really remember a good reason to have it.
OK, so it seems like we have provisional sentiment to strike it. Nice for something like Unix's ls -l. The alternateURL property can cover many of the use cases envisioned for authentication-id, and is safer. However, we should look for more use cases. There are too many cases where there are options whose purposes aren't well specified, and people who guess differently run into trouble.