Yahoo is the second most important search engine and if you ask anyone in China, Yahoo is the #2 search engine only behind Baidu, and in that country Yahoo is very well represented and has great partners (like Alibaba.com) that keep them in the mindshare of Chinese internet users. Yahoo has about 14% of the UK market and I believe 25-30% of the US market. Its traffic is not to be ignored.
I have my own theory about Yahoo. Yahoo as a search engine is more focused on the business field than on the information field. Yahoo has concentrated their efforts on online stores, real estate, car sales and other business related companies. I looks like their algorithm is optimized in a way to answer queries related with online business, while Google has optimized their algorithm for information in general, since Googleâ€™s algorithm works better in that field than Yahooâ€™s algorithm.