I do not think that ZeroOne is properly expressing what he wants to discuss.
Everyone is speaking of negative SEO. I do not think that is what Z1 was getting at. As an example, many here use AA blogs for links. Over time those blogs become well known spam sites and G devalues any link from them. However, if large domains also had many links from those site, then G would have a much harder time devaluing the links without also affecting the larger domains. This would have the practical effect of mitigating any action G may take to devalue a link from a spam site, and would also make it much more difficult to tell if a site was a spam site or not. Many of those larger domains are paying customers to G, and it is not a good thing to piss off your paying customers.
What I believe the OP addressed was not negative SEO, but rather protective counter measures and monkey wrenches in the gears.