Seems to me the way to know who has the power in therapy is to ask the question--who will suffer if it ends tomorrow? If there is emotional investment on the part of the client (as SM mentioned), then how could it not be the client who will suffer exponentially more?

I have been in therapy where stopping had little impact on me, and that's because nothing of significance had occurred and so it wasn't really therapy at all. In another case stopping was traumatic because I was so emotionally and psychologically defenseless, having dared to develop a secure attachment and to risk openly exposing the full extent of my need for the T (to paraphrase one psychologist).