My answer: never. Here's why…
Disputes of almost any kind always seem to devolve when one party questions the motives of another party.
The funny thing is, it's virtually impossible to know for certain someone else's motive for any given action or statement.
“Thought experiment” to see if I'm right: the next time you think you know why someone says or does a particular thing, stop and see if you can come up with 2 or 3 other explanations for why they may have said or done that same thing.
An even more interesting (and instructive) “thought experiment”: try coming up with explanations that are only positive in the motives you attribute to the other person… and then come up with an equal number of explanations that are only negatively motivated. Compare the two lists, and ask yourself: which ones look like the motives I most often instinctively ascribe to other people?
What did you learn?