How many can name the product from last banner ad they saw? Chances are, none of you.
The Journal of Consumer Research published findings that repeated exposure to banner ads results in positive feelings for that which is advertised. They asked students to read an essay with the pretense of answering questions about the essay when, in fact, the survey focused on the content of banner ads displayed alongside the content. Those asked about their positive feelings towards the brand advertised saw positive reactions consistently increase in conjunction with number of exposures to the ad.
Consider now, as you already should, that banner advertising can not be measured in click rates or CPCs but that the ads indirectly influence behavior. With this continued support of that influence, take a broader look at your site metrics, search behavior, or action rates (conversion, registration) when running display campaigns to evaluate that indirect influence.