Google recently revealed that their click fraud is less than 0.02 per cent so it is barely a problem anymore. This means that for every ten thousand CPC, less than two are possible click frauds. Moreover, Google has algorithms that automatically compensate adwords users based on the percentage click fraud.
Ads pay for Google's multibillion dollar business. They are introducing Pay-Per-Action in order to diversify their income channels, not because Adwords is failing.
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.
Ads pay for Google's multibillion dollar business. They are introducing Pay-Per-Action in order to diversify their income channels, not because Adwords is failing.