If you are going to optimize your site for Yahoo search engines then you have to know what Yahoo loves, it would help you in optimizing your site easily. What I have experienced that directory submission works good for Yahoo and Bing. Also try to include your site in Yahoo Directory and Dmoz, these two would give good benefit without any doubt.
My websites have much better rankings on yahoo, but traffic comes from Google. So, this means what it means.
Well nowadays SEO demands unique content, relevent back links and many more things. Social media is also a big contributor in it now. So perfection will give high ranking in all search engines.
Yes if we optimized a website for Google it will be for all search engine, The unique and quality content is must for higher ranking and higher traffic.
Yahoo, is the second Big Search engine in the world, if our website is present on the top in yahoo, it is good for our website's visitors. content is the more important to achieved your keywords on Yahoo.
With Google hard algorithm updates,it is becoming hard to optimize sites for Google. Optimizing for Yahoo can give quick results and nice amount of traffic to your website.
I didn't receive a single visitor to my sites so far. Most traffic comes from Google and Bing. I will work more to improve SEO and content. Best regards from - Romania