I don't believe it is. First, I don't believe that people should be encouraged to go on believing things that aren't true. Second, it might divert attention from the real source of the patients' problems. Third, it's not fair to the parents. Fourth, it could harm patients in the long run by making it harder for them to maintain or re-establish close ties with their parents.
People seek psychotherapy because they are unhappy.
I agree that we should not send our patients out into the world blaming their parents for their problems. But in my experience, this doesn't happen. As patients improve and become less defensive, most of them spontaneously realize that their parents meant well and did the best they could, given the constraints of their own background. Patients then begin to enjoy a new relationship with their parents in which they feel sympathy, rather than anger, at their parents' limitations. I do not believe it is fruitful to spend therapy time debating the nurture assumption with patients who blame their parents for their problems.