As much as I like the Tau on tabletop, they are just extremely uninteresting story wise. You either get the version of them that doesn't fit into 40k or their "updated" society in which they are all brainwashed drones serving the Ethereals.
Except, in "updated" version, they aren't brainwashed. Farsight still exists, and there are mentions of T'au going against Ethereals wishes. Also, I don't think it's a good idea to say that T'au will be uninteresting, Teen Spirit, mentioned multiple times that he didn't add certain options, such as Magnus, because he had no idea what to do with them or wasn't confident in telling a story with them. So there's a plan for T'au, they will influence the story just as much as would Eldars or Night Lords or Custodes if we choose them. I don't think it's good idea to disregard them from the start by generalizing them as "uninteresting story wise".
Also, T'au have caste-based Society, so there is 90+% chance that our "T'au squads" are solely made of Fire Warriors (and nothing else)
They can also have some other species here, like Kroot or Vespids, and if we are lucky maybe Air Caste pilot and Earth Caste engineer who was repairing stuff on the battlefield. Also, still, they have practical experience, living in multi-species society like I said. Just because they can't give us detailed instructions about how such society worsk exactly from top to bottom, doesn't mean they can't offer insight, in how to cooperate with various different Sapients. Also, they can offer us insight into tactics and way of doing warfare vastly different from Imperium.
Might as well argue that we shouldn't get Night Lords, because...They Night Lords and nothing else.
And to be fair to Kurze, she's also not trying to reform a legion. I'd be willing to bank on the Primarch connection being strong enough to change a couple of squads, especially when they'd be under her direct supervision.
Again, that's what I'm talking about. Primarch connection is not mind-control. Primarchs can't just rewrite the mind of a Marine, otherwise guys like Rylanor wouldn't be a thing. Corax wasn't able to tell Pale Nomads to just change and he had to just get rid off of those who didn't agree with his changes to the Legion way of conducting operations and war.
Again, I'm not saying we flat out, can't change them, I just doubt Teen Spirit, give us a option with Pre-Heresy Night Lords, with our own main character remarking how said group of Night Lords is composed solely from murderers, rapists and thevies straight out of Nostramo, only for nothing to happen, because our character will just rewrite their personalities with our presence. They will be loyal to us, this we also know, but that doesn't mean they will just mindlessly to what we demand from them.
We can change them but it wouldn't be a issue that we can just resolve in few moments. That's why I don't think it's worth to take them, considering our current situation, we have better things to do, than having to rehabilitate post-human rapists and murderers, so they stop skinning people as the go to solution to every problem.