@Peter Trewhitt This is true, and therefore it is said that we never can be sure to have found the truth (in empirical matters, and the truth of this sentence then is a non-empirical or a logical one).
Nevertheless there are all these individuals, and we often quite aren´t able to "understand" what´s going on there. True, this doesn´t mean that we weren´t able to understand it in principle, instead we simply don´t look through the complexity (of, say, an individual patient or a new case). I guess this problem leads easily to attempts of generalisation that doesn´t succeed. So I don´t disagree with the assumption that case reports are of lowest or questionable evidence.
For joke I assumed that in an individual would be something that would not be a matter for generalisation, but nevertheless would be at work there. Then things that are able to become generalised or are already known, would have lost their value for application.
In fact, this even might be in a very very tiny small amount true in the universe, and the constants of nature might not be really constant but changing with time, or even with space coordinates. Then a theory of things in space and time would be true only for specific coordinates and every "case of" (using the generalised term) would be in charge only for one single thing.
Begging some pardon for thinking unnecessarily queerly - but this sort of thinking could be used as well as misused in medicine, when the relationship of patient-doctor is said to be important or even said to be a major player.