People vary widely in their reactions to advertising. I sometimes click ads because I want to check the validity of a claim that's being made. For example, if T-mobile was bragging about their coverage I might check to see if they have more coverage in New Mexico than they did a year ago. Just an example. I also click ads because I'm interested in a product. I use Google all the time and I'm certainly interested in Bing. In fact, I'm going to check Bing right now and see if they haven anything enticing...and thanks for reminding me of the Bird's Eye View on their maps.

But yeah, impressions are definitely considered when buying advertising.