I'd say that highly targeted ads are harmful, as it's more likely to undermine the viewer (eg: political ads tweaked based on current family situation/emergencies derived from data mining). Eg: target people who recently lost loved ones to an attack by one of our times typical "lone gunmen" by ads advertising increased surveillance as a way to mitigate "terrorist" attacks.

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).