The author says;
Unfortunately, the privacy modes offered by browsers are still evolving (several are only available as betas), and none remove all the tracking data users might expect them to block. A tool was created to set and report on different data stores. This paper presents the findings from running this tool using several major browsers with two plug-ins across three common operating systems. We find current browsers are unable to extend tracking protection to third party plug-ins such as Google Gears and Adobe Flash. Some of these require no user prompting under common configurations and even expose tracking data saved with one browser sites visited by a different browser.
The Internet was never designed to provide privacy. Every IP is traceable. Tor is a good but painfully slow, and an organisation with resources can track back and find a person IP. Personally, I think private browsing in browsers is only for those who are trying to hide their behaviour from a family member rather than from any external entity.
An implication of this “private browsing” mode is that it makes parental supervision of a child’s browsing behaviour difficult. An option would be for browsers to implement parental controls so they can be switched off if required.