I can think of a very few, very extreme examples that I've worked on where that might be a registrable amount, but even then, you're only saving on duplicated parent classes/element which would have been taken care of by simple gzip compression.

There are [increasingly few] times when it's just nice to just write CSS and I guess unifying nesting will make that nicer, but I don't think there are tangible benefits over "flat" production output.