I'm not even sure we've seen Knight in BftC recently. He's always been a bit of a non-entity.
I could see him letting her go to help the new Batman, out of respect to his fallen friend.
Just something I would have liked to see. She really deserves a bigger spotlight than the occasional guest appearance. Plus, I'm really partial to the Tim Drake kind of Robin, who's smart and competent enough to work with Batman in a true partnership. Dick's going to need to keep devil child on a leash.