Venus I would say it's not necessarily the attachment itself that causes the problems, even if it's unhealthy because we haven't learned how to have a healthy attachment. The way the therapist handles the attachment is crucial. An unhealthy, overly intense but insecure attachment can be modified and made healthy and secure by the therapist's behavior and the client's willingness to trust the therapist to handle it appropriately even when handling it appropriately feels terribly rejecting to the client.
Sure, there are some T's who lead clients to believe that the relationship is more than it really is, just as there are some T's who try to prevent their clients from becoming attached at all. But it really isn't the attachment itself that causes the most problems -- it's the way that attachment (or lack of attachment) is handled.