Plenty of good reason to do this - mainly to shore up Microsoft AdCenter.

"…The rumored hypothetical purchase price is $2 billion. Forget about that for a moment, the acquisition would make Microsoft much more credible in the minds of advertisers than it has been to date and would also give a boost to AdCenter by association. In other words, brand and other marketers reluctant to commit to AdCenter might be more inclined to do so if they’re already working within the company via DoubleClick and can be cross-sold."

As Microsoft and Yahoo continue to improve, make themselves more attractive as alternatives to Google AdWords and AdSense, it may slow down Google's growth.