I think it depends on what the client is going in to therapy for as well as the T's skill and training in handling it.

If one goes in to work on attachment stuff, yeah I'd say it would be rather hard to remember that rationally one can leave at any time. And, in such a situation, I'd imagine that feelings of powerlessness are likely to arise frequently and intensely -- however, although that's a sort of systemic hazard, I'd say that a T who is highly skilled / trained / competent should be able to make it work in a way that doesn't reinforce (or at least minimizes) the client's sense of powerlessness.

But, if one goes in to work on other sorts of issues that don't bring up the same types of early attachment problems, the sense of powerlessness may not ever enter the picture.

The problem of course is compounded by the fact that one may think one is going into therapy for something else altogether and lo and behold, it becomes or turns out to be all related to attachment and then chaos breaks loose -- especially with a less skilled / trained T.