Retainers Question

I have always been given retainers that just cover my first molar teeth. I recently have been told that retainers are meant to cover all the teeth as this prevents an open bite forming. Is this true? I asked my orthodontist and I have been told that they no longer can help me. Even though I said I am happy to get the same short one or long ones whatever is needed. I will go to another practice but want to know facts so I dont get short handed again.