I think this is the justification for scrupulously complying with grammar and usage conventions even where compliance does not enhance either the quality or clarity of the ideas expressed. By its terms, the justification is limited to contexts where “authoritativeness” matters. Thus, where you don’t care about the authoritativeness of your writing — where you are jotting notes to yourself, writing for aesthetic purposes, seeking to persuade without relying on an aura of authority,* etc. — slavish adherence to norms of grammar and usage is pointless. (This is another virtue of the justification, as it gels with our (…my) intuitions about where attention to form stops being prudent and becomes fetishistic.)
Two open and related questions are, first, how important for one’s authoritativeness is slavish adherence to form, and, second, does the importance of such adherence vary across communities? My sense is that usage and grammar are particularly weighty factors for evaluating authoritativeness in the legal community. Other communities have their bugbears — using “beg the question” to mean “invite the question” in philosophy can detract from authoritativeness (whereas lawyers routinely use it in that sense) — but few accept, the way the legal community does, the legitimacy of judging a work by its grammar and usage (as opposed to its content), and few are as globally attentive to lapses in proper form.
I think the explanation for how strongly the legal community believes that authoritativeness depends on form is that, given our rule of law values, the fewer personal judgment calls appear in the law the more legitimate it appears. The more you adhere to norms of grammar and usage the less you appear to be exercising your own judgment as opposed to channeling objective conventions. This makes your legal writing appear more legally legitimate and thereby makes it more authoritative.
* I haven’t bothered to define “authoritativeness.” Volokh’s explanation is incomplete but adequate for my purposes; one has authoritativeness where “when the argument involves judgment calls — is the case really that similar to that precedent? what are the practical effects of a ruling likely to be? — the reader will be inclined to be moved by the argument even beyond what the purely logical force of the argument justifies.”