An additional benefit is semantic identifiers. For example, --main-text-color is easier to understand than #00ff00, especially if this same color is also used in other contexts.

For my point of view, and I've been annoyingly consistent in this for as long as people have been asking for this feature or something like it, style encapsulation is one of the core principles of Svelte's component model and this feature fundamentally breaks that. It would be too easy for people to use this feature and it would definitely get abused removing the style safety that Svelte previously provided.