Let’s assume I didn’t know about vaccines and I went to ask for advice to someone. How would I know if what they told me was good advice?
I would ask myself, are they an authority on the subject? Where do they draw the advice from? Who says they are an authority? What did they have to do to earn that authority? Do other authorities say the same?
Are mormons authorities on logic? Why trust advice about logic from someone that doesn’t follow logic?
A liar can say that lying is bad. A killer can say that killing is bad. It just so happens that the advice is good, in spite of who said it.