The blank slate idea has been challenged recently and very strongly. For many it actually can be harmful and cause decompensation. It is still used and taught, but many now reject it in favor of more of a relational approach. That approach is supported by attachment research and infant neuroscience. Not that the therapist has to be a caregiver, but that attunement and empathy has to be there at least part of the time. Self-disclosure can let the client know about the therapist's subjectivity, which is something the client is very attuned to usually. I believe the newer models toward transparency in the therapeutic relationship help a wider range of clients. There's research that shows that say working with a Latino population, there is more "rule bending" like accepting gifts, sitting closer, touch, and self-disclosure, mostly not out of theory but from accommodating the cultural difference. But it turns out that there is whole bunch of literature out there that supports these adaptations. They don't necessarily work for everyone. And so I think no matter what the beliefs of the therapist or the desires of the client, what matters most is what is in the general best interest. Not easy to tell all the time for either party. What's good about a good therapist is an ability to make mistakes and repair them. Staying flexible and humble should be requirements for the profession.