The Rimm-Kaufman Group recently posted some fascinating data regarding the Bing / Yahoo PPC trends since the merger (and in fact since a bit before the merger, as Bing’s engine started fueling Yahoo’s searches).
They note that paid search traffic from Bing and Yahoo has been declining steadily since the search engine merger began, falling from 24% of paid search click down to 20%.
They speculate that part of the reason for the decline is Bing’s better policing of bad partners (suggesting that a chunk of the search traffic from before the merger was junk traffic). This is supported in their data by an increase in sales per click — or conversions — bringing Yahoo / Bing finally on par with Google’s conversion rate.
Ultimately the most likely reason for Bing / Yahoo’s diminished clickthrough rate likely has to do with the fact that Bing and Yahoo do not monetize their search results nearly as well as Google does. Google displays several paid ads on their right column, a few at the top, and often the Google shopping feed up at the top as well. Meanwhile Bing will sometimes display just one ad.
Arguably that could make for a better user experience — no one likes searching for something and finding the entire above the fold search engine results cluttered with ads — but it certainly makes Yahoo / Bing less attractive for PPC advertisers.