Grace Provision in the Law

Imagine that you have been robbed or raped. Or, you constitute the family of someone who was murdered.
The criminal goes to court, and you take wind of this. Perhaps you're there yourself.
In this case, imagine that there's something in the legal code of your country called the "Grace Provision". Either the victim or the victim's family (in the event that the victim is no longer alive) can use this provision and appeal to the judge to drop/reduce the criminal's sentence.
What do you think?

Dassault Papillon: Hmm, interesting idea. I think it's a good idea, but on the other hand, what if a family allows a serial killer to remain on the loose?

Dassault Papillon: In what legitimate circumstance would it be a good idea to have such a provision?

admin: How about if you're a rape victim but you decide to spare this person from a decade in prison?

Dassault Papillon: And why would you do that? You go through all this effort to secure a prosecution and then at the end say rape's ok, this person can go free. I can't think of a single good reason why judges, highly trained in interpreting law, should not be trusted in this case.

Probably in a debate you'd see the same sorts of arguments as you see for jury nullification, which I think is stupid too.

admin: The law exists at least in part for the purpose of providing justice for the victim of said crime. If the victim has forgiven the criminal, then why is he/she still going to be sentenced?

Dassault Papillon: It's kinda like how rights to sell your own organs are restricted. There are perverse incentives that could make victims release dangerous people. Justice for the victim does not necessarily mean giving the victim what they want, be it a greater or a lesser punishment.