Or a visiting prof can be someone that Duke does not want on a tenure track. For example they would be the spouse of an important Duke tenured faculty member or the spouse equivalent. Thus while Duke does not have a long run slot for them, Duke can employ them for six or seven years as a visitor without having to later vote on them for tenure.
True enough. And a wrench can be used as a hammer, but that's not really the right tool for the job.
...or to phrase it better, that's a misuse of the tool.