a) They're never good enough. One form of targeted ad appears to be 'if they've bought this once, they'll buy it again', failing to take into account how likely

a) They're never good enough. One form of targeted ad appears to be 'if they've bought this once, they'll buy it again', failing to take into account how likely a repeat purchase might be, which is all sorts of wrongheadedness. It's similar to an uncanny valley effect - genuinely perfect targeted ads could be very useful, but below a very high level of accuracy, they just become even more annoying than random ads b) The data required for ads to be sufficiently targeted is beyond what many of us feel comfortable having companies sharing with each other c) Targeted ads are typically delivered dynamically, raising all sorts of security and efficiency drawbacks