What is the harm of granting unearned forgiveness?

Some people are too forgiving. For example, some religions preach that people should forgive a cheating or abusive spouse or forgive a deadbeat sibling who has stolen money from the family -- and many people do just that. I have even seen some people claim that rape victims should forgive their attacker. Aside from the obvious answer that an unreformed perpetrator may commit the same act against us in the future, what are some other real-life practical harms of offering forgiveness when it is not earned?