Specifically, I'm wondering about pages that have a purpose that is sort of removed from the main site. For example, a privacy policy page - would you duplicate the description of the main page or is it best left alone?
Should meta descriptions be unique for each page?
seo;meta description
Yes, all meta descriptions should be unique, because every page is (or should be) unique.However, not every page really needs a meta description. It's only useful for pages that you want to rank in search results because that's where it is shown. For a privacy policy page, I would simply omit the meta description tag entirely.