I had a Red-Mage in my party right up until the final dungeon. Up until that point they can use most of the best swords and magic (Defenders, Firage, Curaga etc). After that you'll probably want to change them into either a Sage, Magus or Devout. Of course, with your current set up that would leave you three mages and a physical class, so you might want to change it to a Knight or something instead.