Why are mid fielders so important? I always thought it was about the strikers....
The way the game has developed in the last 20 years, a midfield that can both help defend and help attack at a high level has defined every great team. To try and apply it to another sport somewhat....balanced play throughout the middle of the field in soccer has become as important as big men who can stretch the court, handle the ball, and pass out of the post in basketball.
With a balanced midfield, you can always have more men defending than the opposing team attack when you don't have the ball, and have more people attacking then they have defending when you have it (and still being safe against a counter-attack when you do lose it).
Of course, not many players are that well-rounded, so you really have to look at your pieces and make sure their attributes add up to a balanced set-up. Because of Messi, Argentina's leans way in the favor of attacking, so when Argentina loses the ball, there's 3-5 people attacking 2-4 people in Argentina's midfield.
You could get around this by having Messi play a non-central role...but he's not
Messi in those roles. If you play him out wide he's just going to cut inside anyway (and now you don't have any defensive width, so the opposing teams defensive wide players and join the attack without fear) and if you play him up top, he sees the ball less and probably gets marked out easier against physical central defenders.
