We all have experienced the pain of hearing that someone, perhaps even a close friend, has said something negative about our character, appearance, family, etc.
It also seems that often those comments have been taken out of context, exaggerated, or even completely fabricated. Nevertheless, it hurts to hear that others think poorly of us. And if indeed someone did deliberately defame us, that is especially painful.
Why say anything about our ex-partner that could be construed as negative or harmful? The old saying of adding fuel to the fire comes to mind.
Especially would this prove true with children. For better or worse, parents are their children's whole world. Much of a child's self-worth comes from their identifying with their parents. While it may be true that the 'other parent' is indeed a louse, why not let the child learn that themselves in their own time? They'll figure it out. In the meantime, bad-mouthing the other parent can only backfire.
Of course, we do need someone to confide in. Someone to whom we can share our legitimate complaints regarding our Ex. It just seems that we need to be careful in our selection. Talking to our counselor or therapist makes sense. Talking to mutual friends, those we shared with our former partner, does not.