I think it's ridiculous, and more often than not people prescribe such behavior for others but are quite comfortable taking personally that which... personally affects them. Silent forgiveness for the inconsiderate instead of numerous means of improving one's relationship with them. Telling someone that they are treating you in an unacceptable manner is, by definition, taking it personally. And being passive and avoiding such conversations is exactly how unpleasant social interactions persist and grow into a climate of alienation and dysfunction.
> You can't delineate a set of criteria for when to take a statement said to you "personally".
Sure i can. When it affects you, personally. People who disagree with me can't come up with a set of criteria. Without such a criteria, there is no justification of the attitude.
> For one thing, taking something personally isn't a binary attribute, it is far more complicated than that.
Taking something personally is binary in this way: you either ignore the personal effects a relationship has on you or you signal/take action based on those personal effects. All of the complication lies in how one proceeds based on their feelings. Mediation and conflict resolution are complicated yes. But you can't resolve a conflict without signaling that it exists and that you have a personal stake in it. That is exactly what taking something personally means.
There are large-scale social implications. People who are lower in status are told to not take it personally more often than people of higher status. When a boss is emotionally abusive, people are far more likely to excuse such behavior than if an underling is being emotionally abusive. In other words, prescribing to people to passively accept uncomfortable or harmful social interactions without protest becomes a mechanism of power. The effects are alienation, depersonalization, stress, and social dysfunction. That shit infests our society.