If hub scores are an important thing for you to manage, then commenting on comments is important for that reason.
Feed back and etiqutte demand the hubber stay in touch, to understand another writers heart. We reach so deeply at times within ourselves, that it is rare to find another who 'gets' our need to write. Comments, well, they help feed our brother and sister authors' need, just as the word to page feeds our passion.
What is a comment anyway? Possibly what it is isn't as important as what it does. And that may well be what keeps all of us coming back and sharing our words to begin with.
~always choose love~