The primary use case seems to be for popular generic-base styles like Vancouver. Here we probably want to leave out the default-locale attribute to allow for automatic style localization. Allowing dependent styles to specify an overriding value for default-locale would then be useful.
This is trivial to implement in the schema and spec, but it does require that CSL processors (or the apps feeding the styles to the CSL processors) handle dependent styles as more than just simple bookmarks.
See also citation-style-language/styles@93dde24#commitcomment-944012
This is fine with me. I think it should be pretty trivial to implement, and it seems superior to independent styles that differ only by their default locale.
Implemented in the schema and spec: