>This tool seems useful if you buy into the philosophy, but I'm just not sold yet - component based styling leads to a lot of style duplication and makes it very easy for a developer to go "off the playbook" when it comes to enforcing a consistent styleguide throughout an app.

You just handle it the same way you handle any shared code. You can keep files in a file scope, in a package / folder scope, or in the top-level global scope (in a small app that's totally fine). But removing globals-by-default removes a lot of "gotcha's" from working with CSS.