The thing is that said threat, while being theoretically ever-present, isn't always going to sow up in every encounter - it's a much more chance-based bonus. We might get Sith today... or we might not. With Brianna we know with certainty that a guardian, if we deploy them, is
always better than they would otherwise be, regardless of the situation. It's two different types of niche, certainly, but I think Brianna's is more broadly applicable and consequently more reliable.
On a new note that I hadn't even initially considered since I got my case for Brianna out as fast as possible in a panic - lightsaber instruction. The Battlemaster oversees that and sets curriculum (at least in the EU so Voik feel free to correct me if his won't be the case here). Atton can teach, yes, but he's not a disciplined teacher, he's an informal teacher. Brianna by contrast lived and breathed drills in her upbringing, and even taught Meetra in formal (to her people anyways) ways (and also some improper ways but so did Atton
). There's no obvious mechanics for it, but as a result narratively I think Brianna would be far more at home in this environment than Atton. I'm a bit hesitant on this supplementary argument though simply because I don't recall if Voik's explicitly stated this will be within the Battlemaster's purview in this quest.