Sometimes, a site receives qualified search engine traffic due to link popularity. In fact, if a site type (such as a Flash-based site) is unable to have the aforementioned on-page criteria, link development is often the only organic SEO solution.

As a side note, I understand black hat SEO professionals do link development, albeit low-quality, spam link development. They fill a niche. Web site owners don't like to accept the fact that their sites aren't as great as they think, and they fall prey to the black-hat SEO sales pitch: "You don't have to change your site. We can get your site instant link popularity."

Web site usability and link development go hand in hand. People tend to link to sites whose content they find useful and informative. In addition, if Web site owners make that content easy to find through a solid information architecture, they pack one of the most powerful one-two punches: unique, useful content that's easy to find.