The main use case I was thinking of is being able to use an inline SVG, but with external resources inside of it (like say a CSS background image using url(...)) in such a way that it ends up loading that embedded content in a cross-origin anonymous way and blocking all embedded scripts. That way someone can't make requests to CSRF exploitable URLs by setting an embedded images to something like example.com/my-submission/favourite

But also so that setting up a CSS transform: scale(10000) can't take over the entire viewport, it'd be constrained to an iframe-like boundary (exactly like an <img>) but still remain as an inline SVG, sort of like an <iframe srcdoc>. So scripts on the parent/host HTML document can still manipulate it like the rest of the DOM, but the inner <svg> elements are all "inert" for want of a better word.

Actually I don't know off the top of my head what happens with an SVG file inside of a <img> when it references external images (either cross-domain or not.) I know scripts and animations get disabled, so I'd take a guess and say some CSS gets blocked too.

Again I've not really thought terribly hard about it, or if it's actually useful at all, and I'm betting it'd be filled with even more foot-guns than there are right now. I'm just thinking out loud.