If you want to win much more matches your elbow should be bent when you take the racket back, when you are hitting the ball and after the stroke. It works for everybody. There are players like Federer, Nadal or Dimitrov who hit the ball with straight arm but they can do it because they are physical monsters. With bent elbow you can adjust to the ball even if you are not in the right place. Your elbow can be used for rotation. Bent elbow not only in one handed forehand but also in two handed backhand. You can see it all in this video.