I think there's healthy attachment and unhealthy attachment. Healthy is when you care about the person and they care about you, but you aren't crushed or desperate if for some reason you have to let go. I think it's possible to have that kind of healthy attachment to a therapist.

Obviously it's possible to have an unhealthy attachment, too.

ETA: a healthy attachment is beneficial, yes, especially if you have trouble with attachment and trusting people, which I do.