The threat that Orochimaru represents is that currently he has completely unchecked authority. Hiashi can give an order with no guarantee that it will be obeyed which will diminish the power of the Hat.
And my concern is that Hiashi already has what I would consider sufficient methods for keeping Orochimaru in line, particularily his ability to give Orochimaru significant leeway and/or support for his research. He also
did give Orochimaru a pardon which gets him Leaf's support system/his stuff (and if Orochimaru doesn't understand tit-for-tat I'll shit a brick), and
does command the military forces of Leaf and the clans, which
does give him the ability to make Orochimaru's life
difficult if he disobeys. He's also working with Tsunade, and although she may or may not be in Leaf proper I
don't think she'd take kindly to Orochimaru's more inhumane research, and Hiashi may be able to get her to counter Orochimaru if he oversteps.
Though this does beg the question I'd posed earlier, albeit in a different format: Are we seriously going to
allow this to happen, if Hiashi calls us out and refuses us? Say we refuse to ally with Hiashi and Orochimaru goes about ignoring orders (that don't directly affect his experiments, so he'll probably leave to kill any invaders that enter his general area) and conducting his research. Are we seriously going to do
nothing about Orochimaru's
research, given that we have a really good idea what his research is going to entail? We're going to have to put pressure to diminish his research for the sake of the people he'd be researching on anyway.
And I did read your answer to my thought earlier, you said (paraphrased) that we'd just put more pressure against Hiashi with our coalition. I...don't agree with that, though. We'd still want to be putting pressure on
Orochimaru, who is the greater threat; and Hiashi knows that.
There is also the fact that Orochimaru might be working towards goals that Hiashi supports
That's a...good thing? From Hiashi's POV? I mean, I can reason out why we-the-Gouketsu might not want that, but Hiashi wouldn't mind?
Threatening to give Orochimaru even more power should work better.
Hiashi
will call us out on it: he knows we care about the common people (see: our tournament speech) and he knows we'd have learned about his human experiments that Leaf found out about. And...we can't follow through. Or at least, I'd refuse to.
Edit: Also, giving Orochimaru-the-bogeyman more power sounds suicidal from a PR point of view.