I agree it's the exception more than the rule, but that doesn't mean Cueto or anyone else has to have a K/9 over 7 to be an ace. While "ace" is an amorphous term, in recent years, Tim Hudson put up six seasons of 5+ WAR baseball with a K/9 under 7; Brandon Webb won a Cy Young and put up a 7 WAR season with a K/9 under 7; Derek Lowe put up a 6 WAR season with a K/9 under 7.
I really don't disagree with anything you say other than that to even look at Cueto as an "ace" (or anyone for that matter), he has to have a K/9 over 7. In 2008, he had a K/9 well over 7, but everything points to the fact that he's a better pitcher today.