Football is a game of offense and defense, that is how it has always been.
Players who think, that they can justify not contributing to the defensive aspect of the game, simply because their contribution to the offensive aspect is overwhelming are wrong.
Suárez is a great example of this. He scores left and right and assists loads of goals, but he also runs tirelessly after the opposition, when they are in possession of the ball.
Why shouldn't Messi be able to help his team both offensively and defensively?