Sages are casters that can use all the schools of magic at the cost of being pretty terrible physically. It's a very established job, dating back to I think ff3 where it could use black, white and summoning magic, didn't have much mp, and was Noah's job in the story.

While on the surface it might seem to be red mage but better, it shouldn't detract from RDM as long as they try to fit it into another role (healer in this case).