The Simplex is the step that eases the transition to the Graduale Romanum...

Here's proof:

http://www.youtube.com/watch?v=Ie7zFRfmTOU

Once your singers are able to sing these with beauty and grace, they can easily move to the Romanum...having developed a musicianship that is needed to conquer the challenge of getting the ear away from major/minor and into the modes.