"Me too" posts aren't worth posting so if I have nothing to add I'll +1 the comment I agree most with/is most accurate to my knowledge and leave it at that. The result being I have a lot more "You're wrong, here's why..." or "I disagree with [something said]" than I have posts saying "You're right, and also..." since I don't always have something to add. It makes me seem really negative - since nobody can see all the comments I've +1'd. Multiply that by every user on the site and things look more negative than they are.
I use "-1" for "You're horribly wrong and I don't have time to correct you (though sometimes I have time and will)" and "Trolling/Vapid posts". If someone is "a little wrong" I don't -1 them and will just post a minor correction.
But in a discussion where you have an upvote button, there are basically only two follow-ups to an assertion which remain on topic: "you're wrong", or "you're right, and also". Anyone who wants to +1 will just click the upvote button.