I’m not sure about the rest of the world but here in Australia Google just has a strangle hold on the search engine traffic generation. Yahoo & MSN here downunder just don’t provide much traction for search engine traffic and only provide a fraction of your site overall traffic compared to Google. From my experience working on client sites, Google provides 80-90% of any sites natural search traffic with MSN, Yahoo and other 2nd tier search engines taking the rest and that’s even if you have a top 5 ranking on MSN or Yahoo. A few months ago I attended a search engine conference where it was mention by a Google rep that here in Australia, Google has 80% market share.
One of my sites is ranked in the Top 5 in MSN & Yahoo but still only get a small fraction of overall traffic. Even a page 2 Google ranking can be better than a top spot on the other big two. So what good are the other two search engines for? Well I like to use them as Google ranking indicators as I usually find that MSN & Yahoo will give you early indications of rankings before Google will so if you were ranking in 9th position for a search term in Yahoo then it shouldn’t be too long before you see these same results manifest in Google too.
Another use for them is to use them to help you find trends with problems on your site. If you see your site dropping rapidly across all the search engines then you better start investigating as there is a fundamental problem with the site that needs to be fixed.
You might even want to include them on client web site ranking reports because the rankings look good on paper too! I don’t want to bag the other two too much but I have a good feeling that Yahoo is slowing moving in the right direction and might yet put up a challenge to Google’s domination in the future with their involvment in many social aspects of the web. Only time will tell.

You mention that you use yahoo as an “indicator” of where a site may eventually be ranked in Google. That’s interesting because since you have optimised my site, the google rankings have actually jumped before the Yahoo & MSN rankings! Is this because you tailor your optimisation specifically for Google?
Cheers
Hi Tim, nice to have you aboard so to speak
With any SEO work you find every site has it’s own persona and tend to vary in the way they rank. Your site was in fact one of the very few new sites that ranked well in Google before the other search engines and considering it was a brand new site, new domain etc even I was a little surprised at the success we’ve had.
In saying that, looking back at the earlier ranking checks we did on your site, Yahoo was showing rankings for other related keywords other than your domain name well before we got those rankings in Google. Using Yahoo & MSN as indicators for rankings is not cut & dry and isn’t a hard and fast rule but it does provide some insight into where Google might rank you in their search results when your site does start ranking.
Also I always try to work towards ranking in all the search engines because the techniques for SEO are the same. Every SEO has their own strategy for ranking and this is based on previous experience on getting rankings. Every site you do SEO for gives you a little more insight into how the search engines work and you start to see consistencies and patterns in what you do and get an idea of what might be giving you an edge. Even though Google dominates search results here in Australia, you never know when thing may change, plus it doesn’t cost any more to rank in Yahoo & MSN so why not!