A society based on myths and conventions is not necessarily bad, but human nature being what it is, it is very likely that such a society would lack human dignity and freedom. Those with power will use their power for selfish reasons and then we get into the realm of propaganda, secrecy and oppression.
Borrowing again from Richard Dawkins, the best reason to believe that something is true is evidence and not tradition, authority or revelation. Pre-Enlightenment societies maintained their conventions not on scientific enquiry and openness but on tradition and authority.
There is something valuable in being confident that what you assert is in fact true. This means also that you need to admit that you don't know everything! By placing a value on the truth this emancipates members of society, since anyone with a brain and the power to reason can draw their own conclusions about what to believe and what not to believe based on the evidence.
Furthermore, for markets to work efficiently the market participants need to have knowledge, which in turn necessitates that what they know is true. An economic system based on lies and misconceptions is doomed to failure (there are plenty of examples).