This article makes a pretty good case about how much of online marketing seems to be not worthwhile.
There are a lot of details in the article, but this little excerpt best describes it.
Picture this. Luigi’s Pizzeria hires three teenagers to hand out coupons to passersby. After a few weeks of flyering, one of the three turns out to be a marketing genius. Customers keep showing up with coupons distributed by this particular kid. The other two can’t make any sense of it: how does he do it? When they ask him, he explains: “I stand in the waiting area of the pizzeria.”
Essentially it describes how many companies are spending millions upon millions to pay for links that are used by people who would have just used a free link anyways.
Someone searching eBay that clicks the ad link instead of the free one isn’t a conversion.