I can look at a comment like "Democrats are just like Republicans but on the other side." and I will not automatically downvote it, even if I hold all the same beliefs on the topic that you've stated you hold. I am not the author of that simple standalone comment, and therefore I have no way of knowing what the author means (nor do you.)

In context, the statement might be talking about politicians, but it might be about ordinary Americans in those parties. Does he mean they vote based on the same issues? Or that they look for similar qualities in a politician? Or perhaps our Commenter is just pointing out how similar all politicians are regardless of their party, using a common stylistic device known as "hyperbole." I have no way of knowing, so instead of voting, I'm probably gonna reply.

In fact, if I thought all view points were equally worthwhile, I would vote based on like/dislike just as you do. If I believed all viewpoints were equally worthwhile, I would be more trusting that the downvotes and upvotes will average out to make the best content also the most upvoted content, ergo the most visible. (since more worthwhile viewpoint-holders upvoted it than anything else.)

However, since I believe some viewpoints are less worthwhile than others, I will occasionally upvote someone who I personally disagree with. Why you ask? In cases where I disagree with the bulk of a comment, but it taught me something, I want others to gain insight too. In cases where the thread leads to an awesome reply, I'd rather that both are seen than neither (and truth be told I'm willing to risk the occasional guy who reads the comment and not the reply). And since some viewpoints are bollocks while others are legit, if a comment makes me reflect on my own viewpoint and reevaluate the others out there, it deserves to be seen regardless of it's overall correctness, simply on the chance that it will have a similar effect on the next guy and give everyone more legitimate, rational viewpoints

(this was supposed to be a TL;DR, but...):

I don't know much, but I know this: I am human, I am fallible, and of all the opinions I have, facts I believe, and assumptions I make every day, some of them are sometimes wrong. I know that there are people out there [i.e. on reddit], some similar to me in their rationality, others different. All of us are in the predicament of not all being correct since our opinions all conflict. So when I come across something that I think is wrong, I don't automatically downvote. I think. I reply. I read other replies. I go on Wikipedia and look up stuff I don't know about. I forget about it and open my laptop 2 days later and see it and get a flash of insight about a vaguely related thing I talked about yesterday with my buddy. And if reading it was a complete waste of time, I might downvote. But if I gained something productive tidbit of insight from the redditing experience that I think another could find valuable, I'll probably upvote.