I suppose it should be easy to detect whether the old or new format is in use, if indeed we care about supporting Samba 3. (It is 10 years old by now, but I wouldn't be surprised if there are still servers using it...)
At the time this was originally put together, it would have been odd to jump to supporting Samba 4. Even so, under the principle of "greatest compatibility" (Samba 4 still supports the Samba 3 format) I would argue we should probably start with the Samba 3 format and then add read-only support for the new format as a separate patch.