I gave the thought of putting the ball in castles hands a bit, and I may be (likely to be totally) wrong but from a tactical view, given that castle can’t shoot from the outside, he is not going to be an off ball catch and shoot threat, by putting the ball in his hands you are forcing the defence to put a man on him, even though the defender can back off a bit, he still has to guard castle instead of collapsing in the paint or double someone else. Another way of putting it is castle being an initiator is better than castle being an off ball shooter.
As castle develops into a stronger shooter, the ball will start to get in the hands of more players to initiate the offence.