Dude, to be fair, there aren't that many in the NBA who are great at defense. Carmelo Anthony isn't great at defense, LBJ is average, so is Kobe. Even Avery Bradley isn't the best at defense, yes he is a lockdown defender but even he has been poor at times defending.
Turner is more of a scorer and we supposedly lack scoring, so it works out, and Turner can only get better with Stevens and he can get better at defense with him.