I think people walk into a therapy room bringing their own assumptions about people and approaches to relationships. So if a client treats others in "real life" as opponents, and acts like relationships are battles to be won or lost, then that's going to come out in the therapy relationship. So of course it's permitted. It's allowed. It's probably inevitable and potentially therapeutic--if it's examined and discussed.

As for the therapist...yes, the therapist should not regard the client as an opponent. It should not be a battle, where the therapist is like a battering ram, trying to get through the client's defenses, and force change. That's not going to work. Therapy should not feel like a fight. I agree with you that the therapist should not "regard" the client as an opponent.

However. If what you are saying is that the client can oppose and oppose and oppose and oppose, and the therapist can never feel frustration and never feel seriously annoyed, then I would disagree with you. Therapists are human. If a client continually disparages the therapist or treats the therapist with utter contempt, that's going to have a serious effect on any normal human being. The therapist has to contain all those personal countertransference feelings and analyze them and work with them. And probably some therapists are better at this than others. But part of the therapy process is to help inform people about the effect of their actions on others, to give them insight into their relationship troubles. I am not sure I am expressing myself well here, so I'll just shut up....