Is Yahoo still important to my site - if I'm purely B2B?
Absolutely. A link to your site from the Yahoo directory gives search engine spiders another path to your site and improves your site’s “popularity” (the number and quality of links to your site). AltaVista, AllTheWeb, Teoma, and Google all factor in popularity when giving your site its ranking.
So, getting listed on the Yahoo! directory will help you to rank better in searches performed on most of the other search engines. Google specifically recommends submitting your site to Yahoo and to The Open Directory if you’re having trouble getting indexed by the Google spider.