[esteth]

They'll still no doubt be using geolocation to filter results, as well as time of day, browser being used, etc...

At their scale, they can no doubt see patterns in data that make it worth personalising search based on those factors, regardless of wether or not you are signed in.

[marshray]

I'm less concerned with receiving unbiased search results than I am being in control of the choice to divulge my own identity.