Right right - I'm working on a large framework (built-in ember) where everything is components, hence the appropriateness of rems.
That being said, I'm not sure I follow your example about the footnote - I don't see any difference from what you are gaining with ems vs. rems as rems are also going to scale. The scaling would be from the root vs. the parent element which to me seems much more manageable allowing you avoid having to calculate the result of compounding needed to achieve your desired font-size.
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.
That being said, I'm not sure I follow your example about the footnote - I don't see any difference from what you are gaining with ems vs. rems as rems are also going to scale. The scaling would be from the root vs. the parent element which to me seems much more manageable allowing you avoid having to calculate the result of compounding needed to achieve your desired font-size.