I think I know my answer to this- I've realised my definition of "rational" subtly differs from LessWrong's. When you see mine, you'll see this wasn't my fault.
A set of rules is rational, I would argue, if that set of rules by it's very nature must correlate with reality- if one applies those rules to the evidence, they must reveal what is true. Even if skepticism is false, then it is a mere coincidence that our assumptions the world is not an illusion, our memories are accurate etc happened to be correct as we had no rational rule that would show us that they were. We do not even have a role that we must rationally consider it probable.
One of the rules of such rationality is that pragmatism is explicitly ruled out. Pragmatic considerations have no necessary correlation with what is actually true, therefore they should not be considered in determining what is true. The consideration of whether human beings are or are not capable of believing something is a pragmatic consideration.
You claim that skepticism is incoherent. Firstly, this is circular as you assume things to get to this conclusion. Second, even if you take those assumptions humans are capable of understanding the concept of "I don't know". Applying this concept to absolutely everything is effectively what skepticism is.
Applying this concept to absolutely everything is effectively what skepticism is.
But you are not applying it to everything. You have a strong belief in a platonic ideal of rationality on which you base your concept.
Take the buddhists who actually don't attach themselves to mental concepts. They have sayings such as: "If you meet the Buddha on the road, kill him".
You are not willing that you don't know what skepticism happens to be because you have attachement to it. This is exactly what Wittengsteins sentence is about. We shouldn't talk about...
I've raised arguments for philosophical scepticism before, which have mostly been argued against in a Popper-esque manner of arguing that even if we don't know anything with certainty, we can have legitimate knowledge on probabilities.
The problem with this, however, is how you answer a sceptic about the notion of probability having a correlation with reality. Probability depends upon axioms of probability- how are said axioms to be justified? It can't be by definition, or it has no correlation to reality.