Responding, in the spirit of your comment, to acknowledge my agreement, rather than just upvoting invisibly.
It eventually becomes a circle-jerk. That is the problem with forcing replies. Voat is a good example of this.
Voting as a signal to quality is as important if not more important than the comment quality. For example, HN as a vast amount of high-quality submission votes and related comments, but their still are a lot of spammy, off-topic, low-value, etc. submissions and comments. Key is to insure in any system that there's a cost to expression and that cost is relative to the value.
Another related issue with voting is that users don't understand the intent of the interfaces use or misuse it to achieve an unrelated outcome.
For example, some users on HN use flags as downvotes, but that is explicitly not what they're to be used for per the guidelines; those users happen to know the system and that the system weights flags in a way that's comparable to downvotes.