Guessing TheLogothete is using MS touted number which is 'search powered by bing', includes Yahoo (~23% WW and US only >30%) https://www.google.com/?gfe_rd=cr&ei=PGz5VqD0GJGAoAPIrLjABA&...
It would be different if Yahoo users used Yahoo because Yahoo uses Bing, but that's not the case. For example, Bing gets used in some searches because they cut Yahoo a good deal, and Yahoo pays Firefox to be the default search provider. That has nothing to do with Bing's search quality.