I can almost guarantee that, if asked, most users would request that their browsing not be tracked some ad company you have no knowledge of.
I think that depends on how you frame the issue. "Do you want to have your privacy invaded by evil marketing companies?" vs. "Would you help Google to know you better, so it can serve you better ads?"For me, ads are a necessary evil because I don't want to ever pay for every content I consume on the internet. So, If third party cookies are used to serve me better ads, ads that actually interest me, then I'm all for it.
Just this past week I finally decided to move my blog from Dreamhost to a Managed WP Hosting company, and it was all because of their ads following me all over the internet. I'm thankful to Google that it showed me those ads and not something about beer or hip-hop music or macrame or whatever I don't care for.