SEO is definitely not dead, and while Yahoo is a good search engine Google seems to have a little more power behind it.
I find it interesting that if you search for something in Bing and Yahoo!, those search engines will identify related terms and highlight those as being related.
Yeah, yahoo is also a good platform to do SEO. But Google has lot more traffic then yahoo, so people love to do SEO for Google. And as per some assumptions people in USA usage of yahoo is more then other countries. And personally i love to use yahoo for search, as Google has new spam free results with some awesome, outstanding stupid updates.... Today i read it somewhere on web that, Google places showing Expedia as a hotel location in listings. lolz..... Too much creativity is also
There is no such thing as "good" or "best" search engine because imo all of them are good, it is just that the use of those search engines will depend on the users preference. Anyway, I believe it is easier to optimize your site for Yahoo than Google or Bing.