Would the player be used in the first minutes or the last minutes of the game? If the other teams find out, they might foul that player to limit the attempts to free throws - how would the player be protected from that? What would be the value of a player like that on the market place? How would this player rank all time with potentially (if not fouled, etc.) a 15ppg average?
We had this same question about someone only making 1 shot at a 100% rate and we said that was even valuable.
Imagine the last couple minutes of every game you have a guaranteed 15 points (assuming he can shoot 3’s). Throw him in in every clutch situation and you practically win every close game.