I've wanted this a million times (more or less), too. It'd be super nice.
But I think the idea is just that comments aren't supposed to be that important. While it often doesn't carry through to practice, the theory is that a comment would be:
Could you explain why you used x when y is more common?
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In those cases, down-voting doesn't really make sense. They don't necessarily add anything to the post, they're just kinda there. That's what comments are supposed to be in most cases.
Up-ticks become important when a lot of people have separate ideas of what might improve a post, then. If someone says:
It would really help if you formatted you code.
And someone else says:
It would really help if you actually told us what problem you're having.
The second comment is probably a little bit more important, so it deserves the attention that a few up-ticks will afford it. Either way, the comments will both be deleted in five minutes when the poster fixes the issues.
But if we introduced down-voting, it would start to promote conversations in comments, which is strictly not what they're for.
Downvoter of my comment, could you explain the problem?
-1 to your -1. That only matters on leap years.
can you tell I'm feeling stretched for good examples?
So although they'd be really nice sometimes, they're also something of a step in the wrong direction for what comments should be used for. That would introduce a feature, meant to support an unsupported use-case.