Personally, I don't think this is a great change to CSS. I prefer being able to grep for identifiers to see where they're used. Now you can't rely on that - maybe a rule is now covered by a prefix selector.
I suppose someone could have done
[class^="btn-"], [class*=" btn-"]
I suppose someone could have done
which would have equally thrown you off, but I do agree that encouraging this is unfortunate.