I think the theory behind it is that when you're dissatisfied with an organization, you have two options: Voice (speak up and hope to change things from within) or Exit (leave the organization and find or found a new one).

And at least when I was there, ~5 years ago, Google executives really, really wanted you to choose Voice. Because the history of Silicon Valley is basically built upon talented engineers choosing Exit and creating the company that eventually replaces you. Google itself was mostly built by people who did exactly that - its early (pre-2005) employees were practically a who's-who of alumni from Xerox Parc, Western Digital, Bell Labs, JPL, Netscape, Sun, and academia.

I dunno if it's the same today - I chose Exit 5 years ago (and my grievances were a lot smaller than most of what I hear in the press today - I generally enjoyed my time there, I just didn't feel like it was a place where my professional goals could be accomplished), and a lot of the people I see Exiting recently are folks I thought were very committed to exercising their Voice when I was there. As a Google shareholder this makes me a little uneasy, but perhaps it's the nature of a corporation that internal dissent just doesn't work when you get to 100,000 employees.