In my view, this all boils down to the fact that if Google hosts the information, it means they can display adverts. Google presently makes money off of Wikipedia because the broad coverage of topics increases the proportion of queries that Google can potentially show advertisements for, which represents an increase in the capacity of the market. Even better than that would be for Google to host the information that constitutes the long tail of Wikipedia. Not only can adverts be purchased next to the results for these topics, but if Google is the number one result, which they will be (they emphasize in the blog post that "we are quite experienced with ranking web pages"), they also get to show advertisements next to the result that is clicked on in the event that the advertisement shown next to that result is not clicked on. This is an increase in impressions, and the larger the number of impressions, the greater the chance that one of them will be converted. Even better for Google is when a surfer clicks an advertisement in a Knol article which lands them on a page displaying Google ads, which.... you see where this is going...:)
That's what happens when you leave 10 billion dollars laying on the table. Someone will pick it up. And, in our case, everyone will. However, those who have picked up may also give back.
Fred