Lots people think that Google is the best search engine, and a few search engine experts over the years say that users tend to think Google results are better.
So people will thinking if the result with Google logo will much better then result from Yahoo or Bing logo. Is this just a branding? How if people are just used to thinking if Google is the best search engine? You can try BlindSearch tool to answer all that question. This tool provides results from Google, Yahoo and Bing in three columns but doesn't tell you which column is which search engine.
You can see what happens when you remove the branding from search engines. Is Google still the best? Check this tool out for the answer :)