I will just say theory aside, in practice, this has worked out great for our team. In my 5+ years experience, I've constantly run across the issue of legacy code with thousands of classes, referencing potentially non-existent markup and countless one-off tweaks. You can start with the best intentions and organization in your SASS files but eventually things start to get really crufty. Keeping everything in components and keeping styles in the markup alleviates a lot of that.

You just made me realize this is why I always have a hard time every time I try a CSS frameworks.

It always goes the same way. I'm first amaze by all what they offer, how simple and quick a great UI can be made. Then when I try to use it, there's always something wrong and I hate how it goes and I always stop and goes back to the logical ways to do it entirely in CSS.

I guess I should try some CSS preprocessor, they must support heritage of CSS class I guess.