I think it's very helpful for someone to point out that the source has been shown to be unreliable before, and we should wait for more verification from others knowledgable in the space.
Agreed. I think there's a blurry gray line between pointing out a potentially unreliable source and a lazy dismissal, but if there's reasonable doubt I think it's good for HN. If the doubt isn't reasonable, it will be torn apart by other commenters, and then it's an explicit discussion that people can read and decide on
If you give such comments a lot of credence without doing that own verification then you open yourself to what is essentially a social denial of service attack.
It's really popular online. I think that's because many people here read a lot of this content but don't actually have the skill or background to do analysis. So they give us history rather than examination. Which has some value, I suppose.
The "hey this may or may not be true so someone go figure it out" is lazy, self-gratifying and pointless.