It’s not all about traffic. It’s about conversions. But it’s hard to get conversions if you don’t have the traffic, and while Google is one of the best potential sources for traffic, Google has other search engines besides web search that people use all the time, and it will not hurt to rank in them too.
http://www.webpronews.com/topnews/2009/04/01/tons-of-tips-for-ranking-in-5-other-google-engines