Historically, the claim of consensus has been the first refuge of scoundrels; it is a way to avoid debate by claiming that the matter is already settled.
-- Michael Crichton 

Kinda like "everybody knows" we only use 10% of our brains, even though it is pretty obviously false. Unless you want to argue that neurons only account for about 10% of the brain matter... but there is no unused-yet-usable 90% waiting to be tapped, as those who recite the 10% theory believe.
I think the second refuge is that of "that's the way it's always been done". Acting as though tradition makes something right is rather absurd. Racial discrimination, sexual discrimination, raping the enemy's women during war, stoning heretics: all good examples of bad traditions that we have (mostly?) given up. Or at least given up pretending are okay.