(assuming it's an inexperienced instructor)
Even if its an experienced instructor then there is no excuse not to be taking control in the correct manner. There is the case that 212man says of a small input or restriction - but the only case I could think of where I might use this is TO/Landing when you may be able to correct a small error without fully taking control, at higher altitudes there is time for the students error to develop more without becoming dangerous and is usually a better learning experience.
