https://news.ycombinator.com/item?id=2434975
http://news.ycombinator.com/item?id=2403696
https://news.ycombinator.com/item?id=2677581
etc. Search will turn up a few other threads discussing this issue over the years.
Not saying I agree or disagree with that one way or the other, and my memory of all of this could be a bit off by now. But that's approximately the way I remember the discussion going years ago.
Ideally, someone upvotes a comment because the comment is thoughtful, not because the user agrees. While obviously not everyone does that, enough do.
The EA Forum has two pairs of buttons: upvote/downvote (quality) and I agree/I disagree. Here’s more on that: https://forum.effectivealtruism.org/posts/2JQQZevGENbChSA8k/...
Brigading but also, people tend to associate votes with something being correct (like reddit). More important is the discussion, because there is no guarantee the people upvoting something are actually qualified to say it is correct.
That issue has been discussed and decided.