To be honest with you, the guy is probably right about parasites, such as tapeworms and flukes, not being "nature's janitors". The only stuff I've ever read suggesting this is AV's stuff. I have never read any scientific literature supporting the hypothesis. However, mammals (including hominins and their ancestors) and their immune systems have been exposed to helminthic parasites (tapeworms, roundworms and flukes) for many millions of years. There is some scientific literature suggesting that the extent of this exposure has resulted in mammalian immune systems that actually require helminth infections in order to function optimally. The presence of these parasites in the body could prevent autoimmune diseases and allergies. The parasites could also be viewed as anti-inflammatories.
You might want to check out a few scientific articles on the subject (these are in no particular order):
Zaccone P, Fehervari Z, Phillips JM, Dunne DW, Cooke A. Parasitic worms and inflammatory diseases. Parasite Immunology 2006 Oct; 28(10):515-523.
Elliott DE, Summers RW, Weinstock JV. Helminths and the modulation of mucosal inflammation. Current Opinion in Gastroenterology 2005 Jan; 21(1):51-58.
Jackson JA, Friberg IM, Little S, Bradley JE. Review series on helminths, immune modulation and the hygiene hypothesis: immunity against helminths and immunological phenomena in modern human populations: coevolutionary legacies? Immunology 2009 Jan; 126(1):18-27.
Rook GAW. Review series on helminths, immune modulation and the hygiene hypothesis: the broader implications of the hygiene hypothesis. Immunology 2009 Jan; 126(1):3-11.