People use search to find something to buy, but sometimes, they use search just to find information.
Yahoo has a "Paid Inclusion" feature, which lets sites appear in the natural results for a fee. This is great for Flash-only sites that have no hope of getting up in the natural SERPs, but it also allows sites that are just trying to sell things to get higher up in Yahoo's natural SERPs.
So, for some competitive terms, ALL of the SERPs on page one of Yahoo are either PPC ads or Paid Inclusion ads. This might be OK if I want to buy something, but if I just want information, there is a strong likelihood that the page one SERPs in Yahoo will be filled with irrelevant ads, and do not contain the information that I am looking for.
This will cause me to switch to a different search engine. And if I find what I am looking for there, it will bias me towards using that search engine in the future - whether I am searching for information OR buying.
And this is exactly what is happening in the industry. I use Yahoo every day as a portal for information and for email, but when I want to search for something I switch over to Google because I am biased to that search engine. Every time I try Yahoo Search for information, I am disappointed because their SERPs are NOT relevant to my search term, and I invariably switch back to using Google. So when it's time for me to buy something, I stay with Google's SERPs.
This is the real danger of using Paid Inclusion to get into the natural SERPs. It may bring a momentary gain in traffic, but as more ads creep up in the natural SERPs, more people will switch to a different search engine. And, it's one of the reasons why Yahoo fails in its attempts to better "monetize" its search engine.