Use case: working with zero-runtime CSS in JS libs that supports css extraction like [astroturf](https://github.com/4Catalyzer/astroturf), [linaria](https://github.com/callstack/linaria), upcoming [compiled](https://github.com/atlassian-labs/compiled) and so on. I've already filled a [PR](https://github.com/styled-components/vscode-styled-components/pull/230) ([released](https://github.com/styled-components/vscode-styled-components/blob/master/CHANGELOG.md#0031)) against `vscode-styled-components` extension to support syntax highlighting for custom at-rules: 
