Wait, Jiang wasn't saying that he should stay behind where he was.  He was saying that he should stay behind them and move together. "Stay near him as we move forward." Keep the important person behind them as the others move forward.  He just asked him to stick to Takumi as they move.  Like you have your first line of defense, if these people attack and get past that then they still have to get past Gin to hurt Takumi.  As of right now, Jiang is beside Takumi and suggesting that you all go forward with them right behind you.