We came up with the requirements based on talking to customers and working through the open source implementations of MySQL and Postgres as well as the specification in Oracle.  This approach seems to allow a wide ability of expression without adding multiple interval types or over complicating the representation.  Using these three factors allow you to express all standard ranges.    This can express the difference between any two timestamps.  However, it does not express the difference between FINETIMEs.  That being said, I think FINETIME is more rarely used (and we don't plan on supporting in the foreseeable future).