From the exchange it appears that Google thinks the deal will broaden its network and create increased appeal for advertisers—and then there's the unmentioned retention of AOL's traffic and thwarting of MSN. Mayer suggests that display/graphical/rich media ads will appear on Google sites other than Google.com pure search results. And as many people have already read, Google can sell the 5% stake as early as 2008 in an IPO (or perhaps back to TW!).
Google is making a push into display advertising (read: attracting branding $$), which is a definite area of growth for them (if they can do it right). And AOL may help in attracting certain categories of advertisers.
Search is often touted as a branding medium, which it certainly is in part, but most national advertisers and, especially, the agencies aren't convinced. And Google doesn't (yet) have as credible an offering for brand advertisers as, for example, Yahoo! does.
The deal also could boost Google Talk by, down the line, making it interoperable with AIM (the market leader). But AOL may be wary of giving away too many goodies (e.g., TW video archive) lest Google benefit too much by the deal.
There's also a link at the bottom of the Battelle post to another Mayer interview with SEW’s Danny Sullivan.