My issue with this metric is that it is completely irrelevant, I see users posting replies like "lol, right", "me too", "hi", "you're welcome", and so on. Which are fully useless, add nothing to the topic, yet increases the activity.
No they don't.
No what ? They who ?
Please read your quoted text, then my answer.
If after doing that it's still unclear, well, you're talking about replies increasing the activity.