Advertisers see incremental value in advertising on their brand traffic for many reasons including directing traffic, controlling messaging, etc. plus in some cases have seen incremental gains in brand traffic. See this blog post for more details.
http://www.rimmkaufman.com/blog/the-cardinal-sin-of-paid-sea...
Therefore what is interesting to me about combining a paid brand banner ad and organic links is the limited control the advertiser has in directing their brand traffic or controlling the copy whether the headline or site links. They also lose the ability to report on that traffic as paid brand traffic which is probably fine to the advertiser as the traffic is then reflected as a corresponding increase in organic.
Again to spankalee's point it is an experiment so I am sure the functionality will develop over time.