As the great diagnostician Charcot was fond of saying, 'ça ne l'empèche
pas d'exister'. There is one occasion on which this may indeed be true:
where the PSU is a group of people who form a team or social unit
characerised by a division of labour. A family, for example, or a
community mental health team, will combine people with different
specialisations and points of view. Two people randomly sampled from
within such a PSU will be more sharply differentiated than two people
sampled from the population.
imply that people within a cluster (or PSU) are more different from each
other than they would have been had you sampled randomly (i.e., not by
cluster). Intuitively this makes no sense.