I agree with the others. The only time I've known people do such courses after a PhD is for a career change (e.g. a PCGE to go into teaching). Either that, or they do one to attain recognition from a professional body which they would otherwise not have, even with a PhD (Psychology is an example - people can have numerous quals in the discipline, but if they are not from British Psychological Society accredited courses, the society does not recognise them, in which case further study is necessary to gain registration.) These are the only types of circumstances in which I would recommend undertaking a further postgrad diploma/masters.