It works even if you have low SP, if others agree with you and concur with your downvote by also downvoting. We see that in practice quite a bit when it is clear that content is abusive. Maybe we'll see even more of it after HF21.

I believe you are sightly misunderstanding my point re. reward disagreement and downvotes. I'm not suggesting that the issue here is reward disagreement on the comment itself, but on the post, and the comments are a way to discuss it. If the poster has unilateral control (including ability to suppress opposing views) over the comments, then it obstructs the ability of everyone to assess reward-worthiness. This is precisely the issue with @cheetah and @steemcleaners but I don't think it stops there; any critical opinions are potentially important and allowing the poster to unilaterally hide them is dangerous.

The suggestion to downvote the comments is purely for visibility, but done in a way that requires stakeholder consensus for the comment to be hidden and stay hidden. IMO this is the most sensible approach when comments, particularly critical ones, may weigh on stakeholders' assessment of the post.

I suppose it is true that the two concerns are currently bundled, and they could conceivably be unbundled (allowing stakeholders to vote separately on visibility and rewards) but that would certainly make things a lot more complicated and probably confusing. As things stand now when you downvote content that isn't being rewarded, or minimally so, that is implicitly for visibility, and upvotes on currently-hidden comment is also mostly for visibility (to vote in favor of restoring it).