At around 16% of the search market, Bing has a long way to go before it’s on par with Google, but it’s been expanding pretty consistently over the past few years. Websites that are not optimizing for Bing are not reaching a good chunk of the search market who uses Bing. Here is an excellent interview with Duanne Forrester, the Senior Product Manager at Bing and the company’s counterpart to Matt Cutts at Google.
This interview covers a lot of key concepts in SEO, including: how often content should be updated to be “fresh”; the role of social; how the amount of time spent on a site (or “dwell time”) affects rankings; and how factors like the length of an article or the number of pages that a website has do not “necessarily” contribute to its quality. Although many of the ranking factors used by Bing and Google overlap, the companies have separate algorithms and may weigh certain elements differently. This is why you will usually see different rankings for the same website on each search engine.