...many of his shots are inexcusable, and don't "help the team". A lot of his game is based on the fact that he is more athletic than practically anyone in the league (which makes him one of the best, and most fun players to watch) ...but there comes a point where he tries to do too much which leads to diminishing returns. You look at his stat sheet and say, "he had 11 assists, he was sharing the ball" ...but then you see he shot the ball 30 plus times for a poor percentage. He handles the ball the entire game and is very aggressive - those are good things (especially, as mentioned, when he can get to the FT line), but when your aggression leads to trying to shoot a three over two defenders, that is not a high percentage shot, and is not going to "help your team" especially when you are shooting .277 from the three point line.
As far as his overall contribution on the game (rebounds, defense etc) - it is off the charts good, which is why his PER is so good! But he definitely hurts his team with his shot selection.