So I don't think it's so much a question of understanding attachment theory per se, or of working in an explicitly attachment-based format, so much as it is a question of whether the T is able to help a client feel comfortable and secure, regardless of the attachment stuff they are bringing in to therapy with them. In other words, *not* traumatizing their client by rejecting an attachment if it is there and pushing them away.
On the positive side though, for those of us who do form attachment relationships with our T's, it can be an excellent opportunity for healing, IMO. To be able to form a secure attachment, in contrast to the insecure or unsafe one we had as children, can help us make developmental progress and to grieve for what we didn't have before. As well, the feeling of safety we can get from having a stable attachment figure, I think can do a lot to help us heal from trauma. But those things could still potentially happen, almost automatically, with a T who doesn't explicitly work with attachment.
I guess I am interested in my future T understanding attachment though, because in my opinion so many of my most intractable problems stem from my poor attachment to my mom in infancy. And as well, I know I will attach to any T, and it helped that my ex-T understood where those feelings were coming from so I didn't have to explain them too much or feel to weird about them.