buckworks - 11:28 pm on Sep 4, 2012 (gmt 0)
Why is it good to show those on advertiser sites but not so good to show them on Google?
Primarily because users who are in the midst of an active search have a different mindset than when they see an ad in the context of visiting and reading web pages. The response patterns on search are significantly different than content.
It's worth noting that while Google SERPs don't get IBAs, other properties such as YouTube, do.