Firstly, thank you for the post. I agree most of the points you've mentioned. However despite that, he is a local player who understands more about the club. This is important for every player namely the first 11 to understand and appreciate the history of the football club which gives them the drive especially for example, when it comes to United Liverpool rivalries.
Also, his contribution is unquestionable and always make it hard for the defenders of the opposition team. Similar to Ji Sung Park. So, perhaps he is part of the United strategy leaving the others like Van Persie for instance, to focus on finishing.
In short, every player has their own strengths and weaknesses which ideally complements each other to form the right balance and chemistry. Hence, it is a squad game.
Hope that answers your question