Perhaps you were being sarcastic and trying to illustrate situations in which someone downvoting is useful. :)
If not: Just because you have the freedom to do as you like, that does not mean that your decision is automatically correct. There are good decisions and bad decisions. If no one was allowed to comment on other people's decisions, sites like HN would have no comments at all. In fact, all discussions would be discouraged.
Just because someone else wants to use their vote to indicate "the comment has merit" doesn't mean I can't use mine to say "I disagree". I wasn't being sarcastic, nor trying to be witty. I'm saying the vote buttons are on my screen, they are mine, I'll cast them how I like.
Use it however you want. However, don't post a comment if you aren't willing to explain why you disagree with a well thought-out post.
It's my vote. I'll use it to mean what I want.