Like GL said, that is a loaded question. I myself have been debating over it for the last couple of weeks. IT comes down to this. Do you live in an old house in a polluted part of the country, where your water is contaiminated with run-off from farms and factorys? or do you live in a relativly new house with no contaimanents in your water?
Old house the the copper pipes are staining your water (mine are) and you dont want copper in your system. Farm chemicals and others, can cause havock on your tank if not removed.
That being said. There is some inital research that some types of animals, mostly Nems, that do better and live longer in tanks that dont use RO. But this is just the intial phase and we dont know enough about those critters to positivly attribute one way or another. It just happens to be a coincidence.
P.S. I apologize for the spelling, Im very tired and am not really paying attention to what i type.