O.M. Strategy recently published an article entitled "Why Do Marketers Buy Banner Ads?" and it makes some valid points to dispell the notion of display advertising being annoying and ineffective as a medium. Though it covers off on the primary points, I think the article misses an important reason that banners should be included in your media mix: the causal relationship between banners and search. Though someone may not initially purchase via a banner, the presence of that placement is likely to increase search volume on your product down the road. AdWeek (via an Atlas study) put some numbers around this relationship in an article last year, "The Atlas Institute, part of Internet ad company aQuantive, found users exposed to search and display advertising convert 22 percent more than those who only see search ads". In closing, whether they are being clicked on or not, banners provide visual cues that inform purchase decisions.