I'm curious what you think the exact purpose of clicking on an up or down arrow is supposed to express?
There is nothing in the guidelines or faq to actually say what the purpose is, and I could see an argument for "quality" being the criteria, but there's nothing wrong with using the arrow to express agreement or disagreement, from what I can see.
It may not be "wrong" per se, but it could easily have the unhappy result of making dissent invisible (or at least less visible). Unpopular opinions, if expressed honestly and well, ought to be encouraged.
There is nothing in the guidelines or faq to actually say what the purpose is, and I could see an argument for "quality" being the criteria, but there's nothing wrong with using the arrow to express agreement or disagreement, from what I can see.