Or substituting in first names of friends in paid content ("<person with same name as your friend> give the following reasons to vote x!/buy this").
Not to mention that if advertising works - then it undermines the idea of a free marked where purchases are made by informed independent agents (I think that's a fantasy anyway, but it seems to be a somewhat popular idea).
Is there anything inherently bad about a stalker passively observing and recording your every move, as long as they don't disturb your daily routine or pass the data along?
Even if you find the stalking behavior (cross-site tracking) of ad companies acceptable, there is a significant risk that your data leaks or is stolen. So, I should be able to decide who I trust with saving intimate data and who I don't.