Also, Google outright rejects some dmca requests and Google also releases data about the actions of copyright owners and DMCA activity on http://www.google.com/transparencyreport/removals/copyright/. So even if not all abuses were caught they would at least be published for public scrutiny.
I don't see how the amount of C&D notices correlates with value to users. Google does not appear to be benefiting its users in doing this.