This is difficult to answer because I see mediation as the magic wand for conflict. Conflicts between friends or people who see each other regularly: work together, in same class, neighbors, play in same park, etc. These conflicts have a tendency to escalate if not handled properly.
It is not realistic to just say, "We will not speak to each other again, ever." But, during my 25+ years of mediating, this is the most common response. This is discussed in Lesson 7: Writing the Agreement in our Youth Peer Mediation: Train-the-Trainer Manual and our Everyday Mediation for adults.
Other cases that should be mandatory are conflicts with multiple disputants and conflicts where there are imbalances of power. When there are multiple disputants, the conflict takes on many "lives of their own." Power imbalances such as 2 disputants against one, supervisor against subordinate, 12th grader against 9th grader, teacher against student, etc.