I do not know of any internal usage, but there could be one that I simply
haven't seen. I am pretty sure the href attribute was added originally for
the reasons you suggest - to provide a reference to more information about
the author. I think that the other linking attributes were added more
recently under the proposal that made sure anything with @href also took
scope/type/format.
Michael Priestley may have a more solid memory about why @href was added
there, but if so he'll have to speak up when he is back in the office.
Robert D Anderson
IBM Authoring Tools Development
Chief Architect, DITA Open Toolkit
Eliot Kimber