People will click on clever ads but may not have an incentive to purchase or interest in buying once they arrive at your site. Conversely, ads that properly identify and entice customers in the right demographic that are poised to purchase will have extremely high conversion rates. The goal is to purchase, not to distract. Such referred customers have a high conversion rate, and therefore the blended conversion rate rises as word of mouth awareness rises. Recognizing further leverage, sites with good user interfaces that are fast, convenient and have great customer service are much more likely to capture one's attention and therefore be mentioned via word of mouth.
Success begets success.
No other single metric captures so many aspects of a high-quality Web site in a single number. As they say, you can bring a horse to water, but you can't make it drink. It's easier to advertise effectively for a Web site with a great conversion rate than to effectively convert a customer who happens to have viewed an interesting ad. This is because conversion rate incorporates the total user experience, and advertising metrics alone do not. There are a few, but nothing that should preclude you from making this the No. 1 metric in your company.