[js2]

Relatedly:

Ultimately, the problem is that blocking third-party cookies doesn't really buy you much (if any) privacy from folks who are motivated to track you. On the other hand, blocking third-party cookies does break some real use cases about federated identity and web sites that span more than one host name.

We decided to keep the option because it make some of our users happy, but we decided not to make it the default because we don't think the trade-off is advantageous for the majority of users.

If you'd like more information about this topic, you might be interested in reading this paper:

http://crypto.stanford.edu/safecache/sameorigin.pdf

From http://code.google.com/p/chromium/issues/detail?id=51031