First, it's just so very expensive to implement a media buy across a large number of individual sites (reaffirmed by recent Publicis/Denuo study that found execution costs for an online media buy of 15-50% vs. 2-3% for typical offline buy). Buying a specific audience through scale/aggregation is much more efficient.

Second, an audience-centric approach can help brand advertisers more precisely reach the folks they're most interested in. (In fact, it's interesting to see this online approach being used tentatively in the offline world -- Google TV, through partnership with the Dish Network/Echostar, has been offering some really cool capabilities to serve up TV ads to specific demos/households). The caveat is this new approach opens up new issues that need to be resolved, namely providing assurances that media is placed only on sites that are brand consistent/with appropriate content/of an acceptable level of traffic quality.

Net/net, it's imperative that an audience centric approach becomes the norm -- it's foundational for brand advertisers to continue to migrate more and more spend online.