The three is the least accurate shot in the NBA, but 33.3% from three equals 50% from two. Bass' specialty -- the long two-pointer -- is actually the least efficient shot in basketball. If Bass can extend his range, that makes him much more of a weapon, not only in terms of improving his efficiency, but also in terms of opening up spacing for other players.
I agree with you to some extent that having Sullinger focus on threes can be counter-productive, because it takes away easier shots down low for him. However, where Bass is shooting from outside anyway, there's no real downside of him extending his shot if he's capable of it.