just because the ad is not clicked does not mean it has not been seen...
Exactly. Just because ad banners are rarely clicked doesn't mean they're not worth the advertising money. The ad is still looked at, read, appreciated for all its pretty colours and animations, and often, people end up Googling the product/service whether immediately after or sometime after seeing the ad. This is why measuring the efficacy of the ad by the number of banner clicks is sort of bogus.