I do think there is something useful in there being a paid person, who does not react like a friend or family member to one's frustration and anger at their failings. I see nothing wrong at all about being angry at a therapist. I think it is one of the main points of having one - you can be angry and whatever else -and they don't care. It does not affect them, they don't (if well trained and halfway decent at their job, in my opinion) take it personally.
Some people may need to be angry. It is something useful, I think, to get to be angry and even blame someone without them attacking back. It is very different than in my FOO where letting anyone know you were angry at them was a seriously bad plan. We do not all go to therapy for the same reason and we do not all need or want the same thing from therapy or therapists. And some therapists are evil, some are not good in general and some are not good for a specific person. Sometimes, in my opinion, people go through stages of anger, or blame or whatever. And doing it with or at a therapist seems to me, safer and possibly productive, than in real life. I have students who seem to need to do it = at me, at school, at the way law is and so forth. For some of those students, they seem to need to be there until they don't, and then we move on - when they are ready. For some it is quicker than others, for some they forge a different path for themselves for awhile. It may not be how I would do it, but it is theirs, not mine. Sometimes their stance means they will miss an opportunity or a something else. That is life, in my opinion. Certainly something else could have happened (good or bad or just being) if they had been different - but that is always possible.
I don't find it useful to tell others they are doing therapy (or grieving) incorrectly because therapy is X or it is better to do Y, mostly because I don't find it useful for others to say it to me and I don't believe it is true. I do not think there is a universal correct way to do anything.