A few comments:
I can see SD's point here, that the T is the one who works closely with the client, and a third party, through supervision or consultation, wouldn't have that knowledge of the client when offering suggestions. So they could suggest something that really wouldn't help the client or that could potentially be harmful.

One of the other things about supervision and consultation is that it's still ultimately up to the therapist to decide what to get advisement on and whether to follow that guidance. I think of my ex-marriage counselor. Had he consulted with another T or a supervisor and said, "So I'm being really inconsistent in my boundaries with a marriage counseling client. Sometimes I'll tell her we have to keep the focus on marriage stuff, but other times I'll spend 45 minutes on the phone with her just talking about her transference for me, without charging for it," then I imagine, if that other T/supervisor was ethical, they might have said, "Uh, stop doing that. Consistent boundaries are important. And it's marriage counseling, so you should treat them only as a couple, no individual stuff." But I doubt ex-MC would have gone to someone about that because he probably didn't think he was doing anything wrong. Or...he realized he was doing something wrong, so he wouldn't want to share that. So I'm not sure that supervision/consulting would necessarily prevent harm by T's who think they're infallible. (No, I'm not bitter at all, why would you say that?)

However, for T's who DO realize they can make mistakes or who have a lack of experience in a certain area (say, DID or strong transference), then I think supervision/consulting could be extremely beneficial. I have no idea if my current T has consulted with anyone about me. But he has admitted to not having much experience with clients being attached to him--for example, I was apparently his first client (in 17 years) to request a transitional object. I could see that being a case where he'd want to consult with someone more experienced in that area on how to handle it (I know he didn't consult regarding the object, since I requested it and he gave it to me in the same session). He has seemed to improve significantly in how he deals with my attachment, but I have no idea if that's because he sought supervision/consultation or if he's just a fast learner and has figured out by my reactions what is and is not helpful to me (I also give him feedback).

Anyway, back to T's in general: I think consultation/supervision could also be a place to get a reality check on techniques one is using, to get suggestions for other ways to approach a situation, to help if a T feels "stuck" with a client, to address potential countertransference and how to deal with that, etc. But a T needs to be self-aware enough to realize they need that consultation.