I think there are disadvantages to a downvote button in some cases, the current setup allows for every comment to be seen as an equal (as long as it passes the community guidelines), which can be valuable when discussing particularly contentious issues. I guess it's always a choice as to what community you are looking to foster.
(Disclaimer: I'm a dev at the Guardian, but not in an area related to commenting)
(Disclaimer: I'm a dev at the Guardian, but not in an area related to commenting)