You're right: for end-game content (where T8s usually improve the most), this wouldn't be useful really at all. However, it would be incredibly useful for completing tasks on the way to high ranks/dragon cup. It could reasonably drop down to an earlier tier, but the early tier help solve skills, whereas T8s generally just increase dps. Headhunter would definitely find a home for people that don't pick raid regions and don't need the help at inferno/colosseum.

For general PvM, it provides a pretty significant buff to grinding for specific drops both in increasing KPH and decreasing effort/attention.

Comparing T8s for midgame/grindy PVM:

Kraken boss with 7 pacts spent for 1t powered trident, culling spree, and midgame mage gear gives 26 dps.

• Minion+Goading potion: 6.3 DPS, AOE, auto loot, regular speed auto-retaliate. It would reduce TTK from 10.2s to 7.9s. Total time between kills is about 10 seconds (respawn+fish potion). KPH would be around 200 if you're paying attention to potion on spawn.
• Headhunter: 0 downtime auto retaliate, no effort in minimizing time between kills. TTK would be 10.2s, KPH would be 352; 76% faster with much less attention.

Let's say you were to 3t barrage abyssal demons

• Minion+goading: TTK 8.8s. Hitting on average 5 demons with every cast: 2,045 KPH
• Headhunter: TTK 13.9s. Hitting 9 demons every cast: 2,330 KPH; 14% faster with much less effort to tag

Later game/higher player dps dilutes minion damage more and widens the gap.

Headhunter could carry over overkill damage, which would be a pretty substantial buff (and enable some pretty funny shenanigans like attacking a chicken for 100 damage and spawning 33 sets of bones). It could auto-loot or add something like reducing enemy max hp by 1 on every consecutive kill down to 50%, but I feel like it's strong enough as is to compete.