The ones in the back row can't get hit by physical attacks unless the characters on the front row die. That's why, the characters in the back gain HP very rarely. I can't really suggest having back row characters. You can only use a bow in the back row, and bows aren't as powerful as many other weapons. This means a bow-user isn't really a good fighter, and a bow-user isn't a good mage, either, because the bows give your characters some huge intelligence and soul penalties.