It wouldn't be a problem if that 0.02% was evenly distributed across all advertisers, but I really doubt that is the case and it's the edge cases that make the news, cause lawsuits and damage Google's reputation.

An example I'm personally familiar with, at www.iamfacingforeclosure.com a few visitors banded together and started clicking on the ads over and over again. Sure enough, the guys Adsense was canned, and that couldn't have happened with PPA.