Great question!
What I have found through a few years of helping clients rank on Google is that all the small dogs follow the leader. There are so many aspects of optimizing for google due to algorithm updates and changing of times, that optimizing for other search engines seems silly. Optimizing for Google seems to lift my clients on yahoo and bing. The traffic discrepancy is huge though. If 60% of organic traffic comes from google then the other 40% is divided between the other search engines. I am always surprised however to see rises in Google traffic will correlate to rises in other search engine traffic as well. I do also see the occasional duck duck go but most of the people I have talked to in my circles have not even heard of it. So why optimize for it. Its a near guarantee that most traffic is coming form Google Anyway.
