Well, the thing is that Violent Impact is random and Power Through provides control over your exhaust/finishers.
Slayer in general is quite squishy (low toughness, worst detaunt in the game etc) and more often then not - offence is your best option.
Basic 2H SB in t4
The idea behind slotting Power Though, in t4 at least, that you get into combat, buff Rampage, run in, Devastate, Deep Wound, Enervating blow, Deathblow while still in yellow, then Incapacitate and finish the target with couple of Spellbreakers hits in the red zone. That's a very strong combo, even if started in red.
The upside of it is that you have your wound debuff, -100% crit debuff and stun when you want them, while your group covers the downside of permanent red zone with guard.
Not to mention Spellbreaker(high base damage) spam costs only 20 ap and you get +50% auto-attack haste from Power Though, that pretty much covers the 25% damage loss on finishers.
If you slot Violent Impacts you loose that control and your combo of exhausting blows/debuffs is weaker by having 50% chance to drop into the green for 5sec, that would provide the enemy a chance to recover while you build the rage back and deal laughable damage.
VI always irritated me with having to watch my rage-meter and having to guess if I'll have the rage for the ability or not. And in most times you will die regardless of berserk.
In t3 you dont even have the Enervating Blow nor Push for More, so power of Giantslayer is very mediocre, esp. with less mastery points to take heal debuff and other good stuff.