I fully agree with you here, and think pg is utterly wrong in this regard. In the long term, it hurts everyone involved. With enough people downvoting to disagree and upvoting to agree, the community devolves to tribalism and echo chambers, and scarily shifts towards Youtube comment level of discourse.
Upvote or downvote comments, not people.
Upvote comments not because you agree with them, but because you feel they contribute to the topic at hand, for those who read the thread after you did. Especially if they make you stop even just for a few seconds and consider a new point of view, even if you end up discarding that point of view.
Downvote comments not because you disagree, but because they are of low quality (for various reasons) and a waste of time to read, given the topic at hand. Comments you disagree with can often seem that way, and it takes self-control and, shall I say, maturity, to see which is it.
Oh yes, and feel free to downvote this comment if you must, I did hijack the topic somewhat rudely here. :)