As a user, in the current state, it’s important to understand that no matter if you use a proxy, a VPN, or even Tor (with Google Chrome), Google (including DoubleClick) may be able to identify you using […] X-Client-Data. Do you want Google to be able to recognize you even if you are not logged-in to your account or behind a proxy? Personally, I am not comfortable with that, but each person has a different sensitivity with regards to privacy.
Thomas Claburn, The Register