I can tell you first hand that Major League hitters, Albert Pujols, A-Rod and Manny actively focus on lead arm pull to stay in sync with the shoulders.
It's a great drill. Don Mattingly is the one who started that when he started pulling off the ball. It gets the lead arm on plane and forces it to stay through the ball.
The front leg is firm if you don't leak forward through contact.The front leg stiffens at and through contact because the hips turn through that point.At the end of the swing the knee must again be bent to get through the ball.