(For the purpose of this question, assume that I only need to support SQL Server 2008 and higher. Or to future-proof this question, let's say recent, current, and near-future versions.)
The documentation for the
CREATE LOGIN statement says (emphasis mine):
Applies to SQL Server logins only. Specifies that the password entered after the PASSWORD argument is already hashed. If this option is not selected, the string entered as password is hashed before it is stored in the database. This option should only be used for migrating databases from one server to another. Do not use the HASHED option to create new logins. The HASHED option cannot be used with hashes created by SQL Server 7 or earlier.
There's no explanation as to why this option shouldn't be used to create new logins, and if there is a reason, it's not obvious (to me, at least).
The documentation references KB918992, which while a little unclear, describes steps that indicate legacy (2000-2008 R2) password hashes are upgraded automagically. So I can take a password hash generated on 2000-2008 R2 and
CREATE LOGIN ... WITH PASSWORD ... HASHED on 2012(+?) and have the hash converted to 2012 format when the principal logs in for the first time. I tested this does indeed work as described.
The password hash itself appears to have a 6-byte signature/metadata header, consisting of what I think is a 2-byte tag/version and a 4-byte salt. When a hash is upgraded, the tag/version is incremented while the salt remains the same.
The question is then: if Microsoft has already baked in this kind of future compatibility (which I assume will need to be maintained going forward), is there any reason why this method shouldn't be used for creating new logins?