The context of this feature is that users were using comments to react without providing substance. The primary motivation is to reduce the visual clutter of non-substantive comments.
While we can agree about what users should do, users not behaving as we think they should is the whole point here. Removing the -1 reaction won't encourage people to articulate their concerns. Nor will it discourage people from writing comments that explain why something is not a good idea. It simply provides a mechanism for people to register disagreement that is less obtrusive than the mechanism they would otherwise use. A cavalcade of -1's as comments obscures and thereby prevents substantive discussion.
What users wanted was a way to gauge what features are more important, so maintainers could prioritize the important issues to the community. All asked for was to be able to up vote _issues_, and be able to sort. [1]
Instead we got reactions, and it completely missed the mark. They've made it easier to be passive aggressive. Also it lingers on your post, instead of having the unconstructive comment stand on it's own. How does one respond to a down vote, make another comment saying "... hmm?"? It's weird.
Facebook and Twitter provide strong evidence that people will use reactions. I see no reason to expect that user behavior will differ when they want to express negativity. Facebook's decision to not provide this feature has long been rooted on the pretense that people will use it. That is the basis of this thread: that people will use it instead of commenting.
Also, it's really easy to disagree with what I wrote without suggesting I am naive.
While we can agree about what users should do, users not behaving as we think they should is the whole point here. Removing the -1 reaction won't encourage people to articulate their concerns. Nor will it discourage people from writing comments that explain why something is not a good idea. It simply provides a mechanism for people to register disagreement that is less obtrusive than the mechanism they would otherwise use. A cavalcade of -1's as comments obscures and thereby prevents substantive discussion.