Well, if you're going to use ems, you probably should think about compounding the reverse way - don't say, "I want X absolute font size, what scale do I need",

Well, if you're going to use ems, you probably should think about compounding the reverse way - don't say, "I want X absolute font size, what scale do I need", instead say "I want something smaller. In the footnote example, imagine you'd want to include a button. That button will contain text, and that text should be just as large as surrounding text (or a bit larger - it doesn't matter for this case). You want to specify button layout using ems so that the button is reusable both in the small-font footnote context, and in the large-font body text context. Even moreso, if you decide than the footnote font needs changing, your button will scale naturally, rather than needing you to adjust the rem-based scale.