I think, the cases that are mostly discussed here, in which idealizing becomes destructive, are those when the client keeps obsessing about the therapist to the degree that this obsession starts affecting the quality of their life. I've been through this, as many others on this board have, and this is something I don't want to ever experience again. That kind of idealization is an antidote to the very idea and the essence of therapy because there is nothing therapeutic about it. From what I know from my own experience and from listening to others, the vast majority of cases in which idealizing the therapist becomes destructive are those that include intense erotic transference.