Yahoo uses Bing because Microsoft was willing to cut Yahoo a better deal than Google, which happened because Microsoft was more desperate than Google. That's not exactly a good thing and it's not really a meaningful number.

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.