According to Shelby White, keyboard navigation truly should be used on nearly every site because it is a necessary feature for every interactive web experience.

He writes:

When should it be used and why?

The correct answer is always. When you have sites that require you to click and drag to scroll or have pages and pages of content, why not give the user the ability to skip to each pane or project. If instead you require the user to perform a second action of clicking then you’re not making it easier on them. Sure you want the drag to be part of the experience, but keep in mind that the more unnecessary friction you add to each interaction the higher their disinterest meter will go.

Source: WANKEN — The Blog of Shelby White » Why every website should have Keyboard Navigation