From Google’s FAQ on the deal (excerpt):
“Q. How will this acquisition benefit DoubleClick advertisers?
A. The acquisition will give advertisers more targeting and buying options and will provide maximum reach for their target audience, helping them achieve the best returns for their campaigns. Online advertising gives advertisers the ability to precisely target the right ad to the right user at the right time. Working with DoubleClick, we will make online text and display advertising more targeted and relevant for the user and therefore more valuable to the advertiser.”
[and then the following, my italics]:
“
Q. Why doesn’t Google accept third party tags?
A. We don’t do anything to compromise the user experience on Google properties or across our AdSense network. In order to ensure the quality of this experience we did not accept third party tags because we could not guarantee the quality of the ad or that it would comply with our format policies. Working with DoubleClick, we will increase the relevance of ads online so that we maintain a positive user experience while provided targeted ad opportunities for advertisers and increased monetization for publishers.”
FAQ available via here (see 4/13/07 post).