The technique employed above is what we call catharsis, or “letting off steam.” Yes, this can help in the short-run. As you ask someone who just sent such a letter, you might get a report of feeling empowered or relieved. Yet, there is a 25-year longitudinal study by Judith Wallerstein who found that many people who felt unjustly treated in the divorce are still suffering from considerable anger 10 years after the divorce. In other words, the short-term catharsis may not last and may require a stronger approach to reduce unhealthy anger. Forgiveness may be more effective in the long-run, if the client willingly chooses forgives and is not pressured into it.
For additional information, see Forgiveness for Couples.