In my experience, if I'm making progress on my goals, the attachment is beneficial. If I'm not, all things being equal, it is not. It's not necessarily linear progress though, the old axiom that things often get worse in therapy before they get better is worth minding too. I agree about interdependence as well, I and my therapist depend upon each other, as in other relationships. It's just the trading in therapy is for somewhat different things, I trade money and effort for personal fulfillment/healing to my therapist who trades time and effort for professional fulfillment and to make a living.

An article I find helpful, somewhat related: http://www.afterpsychotherapy.com/at...-relationship/