Not the commenter, but I can provide a justification, though I obviously don't know if /u/tallerthanunicorngod feels exactly the same.

It's about retributive vs restorative justice. These sorts of punishments, while maybe viscerally satisfying, don't address the problem, they only satisfy our collective thirst for revenge. The justification for this is only: "This fucker is so fucked up, and did something so terrible, we're going to gouge his eyes out for his crimes. He deserves this."

Consider instead where the attacker came from. Dude didn't choose his genes. Didn't choose his parents. Didn't choose the environmental or biological factors that would, exclusively, alter the microstructures of his brain's physiology, which in turn exclusively guide all of his actions, from what he chose to have for breakfast that day to the actual decision to throw acid. He's culpable for his actions, but if you were in his place, with the same parents, the same genes, and the same upbringing, there's simply nothing left--you would be him, and would throw acid into a young woman's face.

The insight that this dude is just unlucky enough to have been born himself is cause for compassion (not mercy); in realizing that he's a victim of circumstances he did not choose and could not have chosen, our goal for him, and for any offender, should be to rehabilitate him for incorporation into society. This does not mean that you let him walk free. This does not mean that you reduce his sentence out of pity. What it means is that his time in incarceration should take as long as we feel to rehabilitate him to become a reformed, willing, and participatory member of society, and that you shape treatment in prison around therapeutic and educational activities. This whole eye gouging thing is a waste of time and life in this context. In doing this, you only separate someone further from being a functioning member of society, and, from a pragmatic standpoint, in doing so create one more burden on the state's health care system. Just about the only positive outcome from this is that a short-tempered public's bloodlust has been satisfied.

Now, it's important to note that this philosophy isn't one that's been widely adopted, and isn't one that's reflected in many cultures (yet, I hope). In the US, for instance, we hardly take this approach--there's less barbarism, but our justice system is explicitly retributive. So we're neither here nor there. But I hope that we're moving in this direction.

*Edit: grammatical cleanings up