Sue, I'm not sure and that's why I'm asking Polly.
But it seems like we want to be as accurate as possible, and Iris sibirica is a valid, accurate and currently in use species name.
http://en.wikipedia.org/wiki/I...
http://www.itis.gov/servlet/Si...
And this is just one example. There are many plants that need to have the species epithet included. Excluding them makes the database incomplete.