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.
--pg
It seems many people have responded to you though.
Downmodding isn't for disagreement anyway; it's about the kind of comment HN users want, not the stance taken by the commenter. I don't have downmodding power, but I chose not to upmod you because of your flippant tone.
--pg
It seems many people have responded to you though.