Every authority preaches typographical errors kill courtroom credibility, although typos don't refute arguments. Credibility in a legal brief takes two forms. A brief gains intellectual credibility by demonstrating legal mastery, moral credibility by creating a fair-minded impression. These two forms of credibility arise from different sources. Since intellectual credibility comes from demonstrated competence, writing authorities connect the curse of typos with intellectual credibility's loss, but proofreading skill doesn't measure high intellect. Writing teachers may confess they proofread poorly, and some intellectually competitive environments are indifferent to typos. (See, e.g., http://tinyurl.com/cprlsr [writing teacher].)
Typos are "immoral" in a social context of superior/inferior stratified relations; competitive environments indifferent to typos are egalitarian. Proofreading is part "formality," socially mandatory courtesy in upward communication. The judge, courtroom social superior, doesn't deprecate incompetence at proofreading but stinginess with editorial resources, the moral offense of presumption. Dishonoring formalities brings threat, not disdain, explaining proofreading's importance when it meagerly enhances Clarity and Attractiveness.