Why shouldn't a pre-hashed password be used with CREATE LOGIN all the time? (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):


  HASHED
  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?
The hashing method is presumably an implementation detail which may or may not change in future releases (as it has at least once already). They're telling you not to do it in order to absolve themselves of breaking your scripts/automation if you try to run them on newer or older releases. It's partially supported purely to allow for migrating logins.

At some point in the process, you would have to use the clear-text password to generate the hashed password, so I'm not sure you'd really gain much by supplying pre-hashed passwords.