…it is used in defense of a victim. When a community singles out a particular sin and decides someone needs to suffer “consequences” for it, the penal substitution metaphor is the perfect remedy.
“We shouldn’t punish him,” one can argue quite convincingly, “because Christ has already paid the price for his sin.”
A great example of the proper use of PSA in this context can be found in the post and comments here.
Of course…PSA is hardly ever deployed in that way, is it?