van Nisterooy isn't someone who dribbles or strikes the ball, you should realise by now. He's the world famous six-yard tap-in master.
As for Ronaldo, he's good in terms of entertainment, but as you might've seen against Fulham, those players saw one his moves before he could execute it, and he lost the ball. And another time, he was doing all this fancy footwork, and didn't move an inch before having to pass the ball back.
Forlan is immensely talently. He can actually strike the ball. He just doesn't get the Starts he needs.
As for Darren Fletcher, he fits in the Man Utd squad, and since Fergie trusts his ability, he'll only improve.
As Fergie can't play Ronaldo too much, or Fletcher for that matter because they're still too young.