My personal feeling is that reef animals of all types live in a much more stable / consistent and inter-related environment than most other animals. So that they are more dependent on external predation and the dense web of life that surrounds them for survival rather than a particularly aggressive individual immune system. Freshwater fish, for example, exist in environments that can change suddenly and dramatically. So a strong individual immune system is probably more valuable. It is obviously much more complicated than that - but I have noticed over the years that freshwater fish don't seem to need a surrounding ecosystem to thrive and avoid disease. Saltwater fish and corals, however, seem to struggle long-term in more sterile or isolated environments. Some reefkeepers make it work through very rigorous protocols. But a tank like Paul's - filled with everything imaginable, "good" and "bad" - seems a better recipe for success.
So to answer your question - I would be very hesitant to depend on corals or saltwater fish being able to develop immunities and survive diseases and parasites on their own. I think biodiversity and the predation that is part of a more complete ecosystem is critically important. Which is why individual tanks and public aquariums that lack that biodiversity depend so heavily on medications, quarantine, parasite avoidance and disease prevention. And why I don't believe Paul's immunity thesis really holds up.
But that is just my two cents.