It depends, because the other factor which I don't remember seeing brought up is how well he disguises it. If the batter thinks a fastball is coming down the middle and it drops out of sight, it might not matter whether IT ever crosses the strike zone - the bat will. I am guessing there's a tell that comes early in his delivery, and he thinks he can live with it in order to get maximum break on the pitch. Maybe he could disguise the curve for longer, at some expense in the pitch's quality, and still be more effective. I am no pitching coach, however.