It really also depends on the therapy. There are some approaches that encourage a kind of internalization of the therapist. The work to do that sometimes involves something that looks like it might be a "merging" but in fact it is to bolster self-cohension, and the therapist uses himself or herself as a way toward that goal, to be used so to speak by the client. Other approaches don't believe projection onto the "blank slate" is productive and so that is helped by self-disclosure which regrounds the client in what the reality of the therapist is. These are discussed and researched techniques with lots of people using them and refining them. I don't think there is just one rule that applies to all therapists and all clients.