See the hidden comments to this post for an example. Just one user causes the damage directly, but that wouldn't happen to the extent it did without the polite and informative replies of others that fuel the conversation. Good contributions to bad conversations have negative net consequences.

So there is a dark part of LW archives, looking upon which might very well destroy your soul. Fortunately, you won't accidentally see it -- you have to consciously choose to unhide the top-level comment. I suppose the damage is that people are unreasonably drawn to that kind of thing and they are also unreasonably drawn to replying to stupid stuff even when it's obviously hopeless ('someone is wrong on the internet' syndrome) so we have to protect them from wasting their time. I guess I dislike paternalism enough that such an argument doesn't convince me (and less seriously, if someone feels inclined to waste time while they're browsing the Internet, then they are already doomed anyway).