It isn't about "motivating guys" to play hard, but it's also fair to question how much Hardy values defense or developing good defensive habits when he has basically never demonstrated a willingness to reward good defense with more playing time (to be fair though, he'll occasionally punish players for extremely egregiuos mistakes on defense, at least).
That having been said, the Jazz obviously shouldn't fire Hardy. Worst case, he can add somebody to his coaching staff that can serve as the de-facto defensive coordinator.