As a person centred counsellor with experience in peer support work, I believe that the best therapeutic experience is delivered when there is a strong working relationship between the client and the therapist. In my own experience of accessing counselling and mental health services I have often struggled with the "therapeutic blank slate" - when the person I was working with was entirely neutral and non-disclosive. For me, that was a barrier to engaging with the work.
How can I build a therapeutic relationship with someone who isn't giving me anything on which to build a therapeutic relationship? How can I trust someone who doesn't trust me? I often found my therapists to be evasive - which was frustrating! - or worse, a blank wall which felt like an exercise in controlling me through use of a power dynamic.
There are, of course, risks in the other direction. If a therapist discloses something personal about themselves, this may contribute to a client building up an idea that the relationship is not professionally boundaried. Similarly there is the risk that a therapist might not keep full focus on the client in what is their session.