People sometimes warn law students of the importance of careful proofreading and word usage, by saying that errors (or things that are perceived as errors) impair the lawyer’s credibility. I think there’s some truth to this, but the focus on “credibility” is a bit imprecise — it’s not that people will see the lawyer as dishonest, or likely to be factually inaccurate.
Instead, I think a good way of thinking about it has to do with authoritativeness. Here’s my sense of the matter, admittedly an impressionistic one rather than a scientific one:
Rightly or wrongly, something that seems to be a thoughtful written argument by a respectable professional tends to have more than just a logical persuasive effect — it also has a psychological effect. If the argument is well-crafted, the reader is inclined to credit not just the factually verifiable claims in the argument (whether about fact or about precedent), but the analogies and the speculation in that argument, too.
The reader, even a judge (who is therefore of higher status than the writer), is likely to subconsciously treat the argument as having some authority. This usually won’t let the author get away with total nonsense, or mischaracterization of the facts and the law (once that’s exposed). But 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.
As a lawyer, you want to create this bubble of authoritativeness, so you can take advantage of this psychological effect. But slips, even something as small as a typo, or something less small, like a usage error, can pop that bubble.
First, they distract the reader from the flow of your argument; they break the spell that successful rhetoric can cast.
Second, they make you seem less authoritative — sloppy (as with typos) or ill-educated (as with usage errors, or perceived usage errors). The reader won’t consciously say, “Oh, this lawyer doesn’t know how to use this word, so I’ll vote against his client” or even “so I’ll be more skeptical of his logical argument.” But subconsciously the extra support you’ve gotten from your perceived authoritativeness (whether or not that perceived authoritativeness is justified) will have largely disappeared.
This also relates, I think, to why excessive rhetoric (and especially exclamation marks!!!) are likely to be less effective in written argument addressed to judges and to other lawyers and academics. The people that legal professionals see as having the most authority are the ones that appear to be calmly explaining “just the facts” — this is the law (or the policy argument or the response to the other side’s arguments), and it just so happens to help my client.
If you look like you’re angry, overexcited, strident, or even too impassioned you don’t look like the sort of person that we see as most authoritative any more. You thus lose the psychological force that the bubble of authoritativeness provides you, and you’re unlikely to regain that psychological force through your visible agitation or emotion.
I can’t speak to whether the same is true when it comes to oral presentation, especially to jurors and other laypeople, where the emotion might be more effective. But with judges and other lawyers reading your written arguments, I think the emotional force of anger, excitement, and passion is unlikely to be that helpful.
In any case, that’s my tentative sense of the matter — I’d be glad to hear what others who are experienced in such matters might think.