I think that you have to remember that we are dealing with closed
systems, consisting of very very small volumes of polluted water. The
potential for the proliferation of pathogens is such small volumes is
immense. UV theoretically sterilizes all water flowing back to the
system preventing the spread of disease from one fish (tank in a system)
to the other. Some of these pathogens are quite lethal such as Glugea
and some Mycobacteria. You wouldn't want to expose yourself to TB just
to jump start your own immune system. There will still be immune
challenges to your fish. Live food will continually bring in
opportunistic organisms to test their immune systems, so I would think
that UV would be a good addition to almost any tank and would be
essential in a recirculating system.
I'm thinking of adding one to my reef tank although that will certainly
reduce my micro fauna populations.