Obviously you don't watch all the games. I have seen all 81 so far. Harden does play defense, just not all the time. He is one of our best defenders in the post but one of the worst in the league at guarding quicker guards.
That's what so frustrating about Harden. He has the skillset and ability to play decent D, but doesn't bring it every game. If he could, I would be one of his biggest fans.