I personally own a website company that will be paying daily cryptocurrency dividends based on the shares each member in my site has from tasks they have fulfilled towards app development, guide development, site development etc...
In order to develop the functionality of almost all of our products, videos, tutorials and pages, we want to be able to allow important changes to be voted on by members and the public at large, and then have changes made in real time.
Also, we need a staff in charge of verifying that when a member earns a share they legitimately deserve it based off the amount and quality of effort put in. In order to chose who will have leadership positions in our board of directors, AGAIN a true voting system is required.
I know not of a cryptocurrency site that has true democratic voting on the fly, simplified.
But if steemit had a DOWNVOTE button, I could simultaneously link to both scenarios / candidates from our community site and people would be able to upvote who they like and downvote who they dont like.
I guess I could work around this by saying simply vote on the solution / candidate you choose, and hope that the results are accurate, then choose the one with the most "upvotes"... but if the results are close the room of error created by people surfing steemit for curation rewards or bots just upvoting everything to make money curating, perhaps these might muddle the results up.
I plan on doing one (if steemit adds an upvote button) or the other as a workaround EITHER WAY... But in my opinion, facebook has recently added a similar feature and steemit should too. Facebook has always needed a "don't like" button, and while they STILL don't have a "don't like" button, as you can see below, recently they added a haha, wow, sad and angry alternative to "like" and it was a welcome feature.
Steemit is based off crypto and value: I believe if there is an upvote button, it just makes sense that there should be a "downvote" button. Maybe I am wrong, if so oh well. Either way, here's to everyones future in crypto! CHEERS 
There is a downvote- its called flagging the post. It reduces visibility and punishes the poster.
Disagreement on rewards
Fraud or Plagiarism
Hate Speech or Internet Trolling
Intentional miss-categorized content or Spam
I guess Ill tell people to flag, it just seems like this is more of a spam block feature than an actual downvote button for candidates... oh well.
Because this system is based on positivity. Having a downvote button would inevitably make the site negative. There is the option of flagging, but this is used for content you feel should not be allowed on Steemit.
IDEA: I think that a downvote button should be an option the post creator can turn on, but is off by default.