From a pure organic Internet Marketing perspective, your organization's website really should focus on page ranks in Google - both for the volume of search traffic probable (since Google's indexing reach is so large, and their paid content distribution is so broad), and the fact that Google's results are very tied into the overall context of the Internet and relationships among online content, including websites, blogs, social media, discussions, wikis, documents, etc. Google is also leveraged as the core index behind other, 3rd-party search engines like AOL and Netscape).
It's apparent, however, that MSN and Yahoo search are much more focused on the specific content and relevance of the actual webpage, vs. the webpage's popularity or connectedness to others. This may be why, in pay-per-click (PPC) campaigns targeting consumer products (i.e. people looking for specific items), it's typical to get much better results (i.e. "clicks") with a Yahoo campaign, while PPC campaigns focused on generating "buzz", exposure or simply large increases of broad-based traffic (i.e. those searching for information, guidance, resesarch) do better with Google.
Persons surfing the Internet using search engines do have a choice, but typically gravitate towards one or the other based on the kind of "Internet Information Consumer" they are, and have been. Don't use the Internet much, but want recommendations and guidance? Try Ask.com or Altavista. Are you a real newshound, with a predilection towards online portals and email
provided by a provider like MSN or Yahoo? You'll probably use their search tools most. Really into comparison-shopping and deals? You'll probably start with Google, but may also know to use meta-aggregators like Dogpile (which aggregates paid ads along with organic) or Mamma.com. More experienced Internet professionals, looking to focus in on particular topics might use tools offering advanced categorization and analytics, such as Vivisimo ("Clusty") or Surfwax. For those looking for opinion or perspective on particular topics, the best places to visit are sites with Web 2.0 feedback technology, that focus on the topic (or have specific sections). Flickr's an example of a place to find recommended or popular photographs of specific subjects.
Here's a real good index of Search Engine tools to use, for particular purposes. Note it includes not only pure search engines, but pre-built directories with search built in.
The point of this topic for organizations and businesses, is to know your customer, and understand how your digital assets (i.e. websites, documents, blogs, comments, articles, ebooks, etc.) are managed. Digital content destined for publishing and distribution on the Internet, to be indexed by search engines or commented on by readers, should be carefully organized, managed, monitored and tracked for maximum effect. By carefully monitoring how your content is used and what others do with it, it's easier to create a marketing plan that's correctly targeted to the right search engines and audiences. Focusing on Google may get you great traffic, but it may not be the traffic you're interested in.