Where I work, we're using it on a new project and while it definitely requires a lot of learning(and just generally understanding CSS), it's faster to debug with, easier to take care of edge cases, and the total size for our app is incredibly small.
As others have said it's very strange at first. My co-worker introduced it to me a little over a month ago and I remember thinking "how is this maintainable?". After a few hours it started to make sense and now I can't imagine going back to BEM.
Being able to design UIs without opening a single CSS file has made HTML pretty fun and I've found I'm much better at componentizing the right things.
It would decouple your structure(html) from your display(css). The semantic class applied to your html could remain unchanged while you are changing the style. For example, you could switch from using a css library like tachyon, to swapping it out with a custom css without touching your html/templates. You're right though, it is an extra layer of abstraction that might not be needed.
http://tachyons.io/
Most importantly it contains a ratio-based scale: http://tachyons.io/docs/layout/spacing/