Earlier quoted context omitted.
I would expect that the more a search engine knows about me the more relevant my results will be. I have yet to see a compelling argument why that isn't the case. Since I use a search engine for the sole purpose of getting the result I'm looking for as quickly as possible, I am more than happy to make the trade off.
Finally, someone in the tech world who agrees with me... Is it really a bad thing that Google knows you like x product and shows you things related to it instead of completely irrelevant?
"Brad seemed to really know his stuff in that phone screen, let's go ahead and purchase his past 6 months of search history before we bring him in for an interview". (sub in Facebook profile, etc.)
I'm not saying this will happen. I hope it never does. But there are definitely incentives in place right now to make this a reality, and I haven't event touched on big brother's back door access.