Yahoo Search does not use Meta Keywords for ranking
During the Ask The Search Engines session at SMX East in New York, Cris Pierry, senior director of search at Yahoo, confirmed that the meta keywords tag doesn’t influence their ranking algorythms.
Not long ago, Google announced what is common knowledge in SEO: they have ignored the keywords meta tag for years. The difference between Google and Yahoo Search is that Yahoo is still using the keywords meta tag for retrieval purposes.
What happens when a page is using an obscure keyword in the meta keywords tag only, that nobody else is using? Google won’t return it on the SERP for that query, while Yahoo will retrieve the page and show it.

What’s the use of the keywords meta tag then?
As search engine land points out, the meta keywords tag can still be useful to get the page indexed and recalled by Yahoo for queries that include misspellings. Bing also is recommending this kind of use for the keywords meta tag.
Currently only News Sites can benefit from using the optional meta keywords tag in News Sitemaps: Google News will use it to better determine the category of articles listed.
