Attachment is a vital human need, whereas dependency has pathological connotations (to paraphrase something I read). Seems attachment in therapy could be a good thing, if it is a means to and end and not a repeating pattern or trap as you point out.

But how often does this sort of attachment progress to a resolution and the client becoming more autonomous?

Here is one view:
"Ferenczi [famous psychoanalyst] said that the analyst infantilizes patients. Far from helping them overcome infantile problems, the analyst resubmerges them in an infantile relationship in which it is the analyst who emerges as all-powerful".