I think the only time I've ever seen tempered will as useful, is for it's anti-knockback ability more than it's anti-heavy ability. Even then it's exceedingly situational as Leviathan and A3 N/S are the only instances I ever found it useful. Honestly though, I've noticed it's anti-heavy effects have become resisted more and more in later content similar to Shield Swipes pacification effect, unlike Pacification though, anti-heavy effects wouldn't be OP in any point of the game considering it's long cooldown.