There is a broad question here which has been touched on by meta-questions before, including another recent meta-question:
What is the best way to treat spurious claims from sources that regularly produce spurious claims?
Let me tackle that broader question, and present the arguments from both sides:
Why to ban such discussion:
There are many reasons to avoid allowing such claims to be introduced to discussion forum of all forms:
Space travel forums have more interesting things to discuss than whether the moon landings were faked.
Epidemiologists can do far more good for the world, if they aren't bogged down explaining the autism is not caused by vaccines.
Biologists have better things to do work on than explaining, again, that evolution really is a thing.
Solutions for climate change would proceed faster if the climatologists didn't get bogged down repeating the same basic facts every time someone posted an ignorant comment.
If you want to have productive discussions on a subject, you need to staunch the flow of the constant spam-like attempts to disrupt the conversation with delusional theories.
If a site frequently posts ignorant screeds about religions, races, genders, etc., we could expect people targeted by the screeds, and their supporters, to tire of the unrelenting offensiveness and move on to other sites where they feel less under attack.
Suppose there is a false claim that few people have heard of, and we post it here - with no answer - and as a result more people hear it. This is a disaster - we become a vector for falsehoods, rather than a vaccine. [Hence one of the reasons to demand notability for claims.]
Similarly, there have been studies [citation-needed] that show that debunking a myth, poorly done, can actually reinforce the myth in the mind of the reader - who forgets the boring details of the debunking, but remembers the vivid myth. This should be a major concern for Skeptics.SE and all skeptics: if the science invalidates our approach, we need to come up with a different approach.
Why to allow such discussion:
We love mavericks
The concept of the lone maverick, sees that society's consensus view is poorly considered, who does the experiments to show the error, and then bravely stands up against "The Man" and improves the world's knowledge, is a key image for skepticism and science.
If we censor mavericks from proposing ideas contrary to our common consensus, we become "The Man".
We don't want idiots to become martyrs
By refusing to directly tackle bad ideas, we allow the proponents of those ideas to paint themselves as unfairly treated, and suggest that the ideas they have are so dangerously effective, the establishment is stopping them from sharing them.
By demolishing the ideas with evidence, we remove this opportunity.
We want the claims to be challenged
When someone naive tries to investigate these claims, trying to decide whether the denialists have a point, we want there to be Google hits to demolish their arguments. We want the aggrieved parties of the defamatory falsehoods to have a platform where they can present the evidence for the truth.
We are the fire-fighters
While the space-travel, epidemiology, evolutionary biologists and climatologists have important issues to discuss, and want to avoid being burned in the fire of false claims, we are here precisely the attack those false claims (and to confirm the correct ones).
While everyone else is running away from these flames, we are running towards them - armed with our hoses of empirical data, and protected with our suits of citation-needed. Perhaps I am pushing this analogy too far, but dramatic and patriotic music is playing in my head as I write this.
Stack Overflow questions are successful when they are found by people with a problem googling for an answer. Skeptics.SE is also successful when that happens, but for me a greater success is when some nonsense is posted on Facebook or Twitter, and one of the commenters simply posts a link to Skeptics.SE, where we examine the claim with care.
Spurious claims are time wasters. If we can spending the time to resolve a claim definitively, and that can be used to shut down the claim every time it appears, it is worth the effort. It brings down the per-person cost of fighting the lies that are frequently propagated.
In conclusion, I think we should maintain our current policies:
- We have no taboos but we do expect claims to be written respectfully.
- We demand notability references so we can see if this is a commonly believed claim, or just some drunk uncle spouting off.