A question by XcellenceIT: Does Yahoo! search engine matter any more?

XcellenceIT team has a question and I decided I should help them by posting the question. Here goes:
Everybody knows that Google is a leader in search engines. But Yahoo! search engine comes right behind him.
Besides my natural question: since all my acquaintances use Google, who are those using Yahoo! search engine after all? , I have another question:
Is it worth it putting efforts in improving for Yahoo! search engine? Google has Sitemaps, Yahoo! has Site Explorer, but is it worth the time to have your web site inserted in both?
OK, you have to make your web site as much W3C-friendly as you can. But Yahoo! search engine friendly?

Frankly, I don't think it's worth the time, but I'm looking for a second opinion.

Any feed-back on this will be greatly appreciated.