Even if you only get weak of flawed evidence, then you do what you can to make the best decisions given that evidence. No one tries to suggest you do actual bayesian calculations on everything you know, for the simplest reason that it's not computationally viable.

But if your beliefs directly contradict bayes then you're doing something wrong - there's an inconsistency that's likely worth investigating, unless the matter is really minor. It's a sanity check for your decision making, not a constructive algorithm for the best decision.