ALL players have to drop back and defend!
It would be like playing with 10 men if you had one player who thinks his job is only up front.
Every player needs to defend!
Imagine a player who thinks that it is his job just to attack and not defend. As a player, when the ball is lost, its your job to try and get the ball back irrespective of your position. You need to mark your player.
I agree that every player should contributes in defence. BUT it reminds me of that debate we had about Rooney creating goals for other strikers. Some said the striker's roles is not to create goals, but to score 'em?