The problem is that while this might work great on a large scale, it breaks down at the individual level: why would I want all that ad-clicking activity associated with my account / cookie / IP?
It's pretty common now to do things like intentionally tagging the wrong friends in facebook because people are paranoid that facebook knows too much. Even with non-techie people. They're creeped out.
Throwing off retargeting is something a lot of people might want to do.