I think there probably is some idealization, but that isn't necessarily a bad thing. It can act like a bridge to help you take therapeutic risks you otherwise wouldn't. But I think from a T's point of view, it's a fine line to walk: too much idealization can threaten to hide a "negative transference" brewing (and this can be unconscious, so exist despite conscious good feelings). I think this is what Winnicott is actually talking about, as comrademoomoo explained. I think of it like earthquakes: little ones are protective because they release pressure building up and we can build structures to withstand them; big ones are deadly.