Black gets these mechanics because it represents creatures that are able to take a beating and have bits removed without dying. It's on zombies, also. If you were creating a black card that represented ignoring corruption and keeping going anyway, it would probably be some sort of Lich effect or similar. 'Staving off corruption' isn't a very Black thing to be doing - Black would be turning the corruption to its benefit, or keeping going despite the Corruption.
The point Ninjaman is making is that 95% of the time the Corruption effect is completely indistinguishable from damage, which means it's probably not worth the text. The point of mechanics and rules text is to change the way the game plays in some way - if a rule or effect just doesn't do anything to change how the game plays, it's likely not necessary.Yeah and? In 95% of cases that distinction is meaningless. In 95% of cases, you heal for 1 and opponent deals 1 damage and you die. Or an opponent plays a "prevent life gain" card.
It's not parasitic, it synergistic with damage. If you are against a life gain deck, yeah you need to deal 20 corruption OR AND HEAR ME OUT - "prevent life gain". Because it's synergistic with damage.