Downvoting one post is the same as upvoting every other post.

I don't think they're "the same", but I believe I understand what you're getting at by that statement. I myself oppose down-voting and have never used it to the best of my recollection. It goes back to what many mothers tell their kids, if you don't have something nice to say, don't say anything". That principle is well expressed by withholding your vote.

That said, there is value in expressing a contrary or negative opinion or perspective in the right context and without aggression or malice. The exchange between you and @dwinblood has value to the community and is a model of respectful communication of contrary perspectives.

Down-voting may be beneficial to the community when a user is "clearly out of line". Of course being "clearly out of line" is highly subjective. I would draw that line at when harmful aggression is initiated but that's just my opinion. I can also empathize with the perspective that down-voting can be a useful tool to ostracize "bad" people from the community that rarely if ever contribute anything the community values. I would hope those cases should be more rare than common however.

Also, doesn't down-voting negatively affect a user's rep score? If so down-voting is definitely different from simply not voting at all which has zero impact on the rep score. Whether or not down-voting benefits or hinders Steemit's platform reputation is not obvious to me, just as I can't say that it improves or weakens the accuracy of an individual's rep score.