I think the key aspect of Heart of the Fury is that carnage still applies on those attacks, making it potentially able to clear a group in one hit.
Imagine you're a barbarian attacking 5 enemies in a pentagon around you, situated such that a carnage attack on ones hits the two adjacent to him. Heart of the Fury means that not only do you strike each foe once, but the carnage on those adjacent them to is applied, leading to each enemy being hit by one attack and two carnages instantly. Higher intellect could lead to that being exponentially more damaging. But, as with all barbarian abilities, its effectiveness scales only with number of enemies nearby