Yeah, Heath is right. If you employ a defensive midfielder solely for the purpose of aiding the defence, then why would you want to have your attacking midfielders defend as well?

There's also the matter of what the manager wants a player to do. For example, Ronaldo may be a midfielder, but when we are defending, you'll find that he is often basically given the role of a striker while one of Rooney or Tevez will drop back. This is because if there is any way that the ball should be taken away from the opposition, you want your fastest men far up the pitch so you can quickly get the ball to them and they can then quickly take the ball past the opposition defence and get a one-on-one with the goalkeeper. If there are two players, all the better - two-on-one!

Having said that, Ronaldo does get the ball off opposition - he's just basically a 100% attacking player, so even when defending, he'll not do what most defenders do - that is, boot the ball away from the defender or slide in with a hard tackle to, well, boot the ball away from the defender. Instead, he'll prefer to steal the ball through quick footwork, and then you'll find that he still won't boot it up to the other end of the pitch - he'd prefer to keep the ball until he can either get a goal, make a cross or pass it to another United player. Even if there are three players right around him, he'll probably back himself to get it past them or else get fouled while trying. But yeah, he refuses to simply give away the ball, I've noticed that much, he'll just keep it to United players. After all, if the opposition don't have the ball, they can't score a goal.

The best defence is offence.

Still, every team will generally be able to get themselves into a position where nine of their players can be used in defence. For United, there are two players who I prefer to see attacking than defending. Ronaldo and Scholes. Ronaldo because he's so dangerous on the break, and Scholes because, er, he can't tackle without getting a card.