Are pseudo-patient studies justified?

Pseudo-patient studies are studies in which a medical sociologist or anthropologist masquerades as a patient. Medical treatment is sought without revealing that the 'patient' is really a covert research worker. When access has thus been gained to a medical setting--typically a hospital ward--social interaction between medical staff and patients is then observed over a period of days or weeks. Important studies have been carried out in this way of psychiatric treatment and of the care of the terminally-ill. Is the use of the method justified? What ethical problems does its use raise? How do the undoubted advantages of the approach compare with the clear drawbacks and objections which can be made to its use?