I think it's ok to use the up and down arrows to express agreement. Obviously the uparrows aren't only for applauding politeness, so it seems reasonable that the downarrows aren't only for booing rudeness.
For what it's worth, I think PG is completely wrong on this. The site was designed, via progressive lightening, to censor downvoted posts. His statement is essentially permission to censor dissent and doesn't make for a healthy conversation.
It's also just human nature. People will upvote what they like and downvote what they dislike, and for some that just means downvoting whoever disagrees with them.
I think it's ok to use the up and down arrows to express agreement. Obviously the uparrows aren't only for applauding politeness, so it seems reasonable that the downarrows aren't only for booing rudeness.
https://news.ycombinator.com/item?id=117171