Mari: "No, no, no, Sage's fucking balls, no! After all the work we did to get you and Kei out from under his scalpel, you're going to jump right back in there?"
Noburi: "Not saying it's a terrible idea, but it is pretty insane. Though I guess that hasn't stopped you in the past.
Mari: "Ugh. You're right that the incentives don't line up for him to defect openly, but that doesn't mean he won't do it discreetly. At the very least, there's no way he doesn't investigate your bloodline while he has the chance, and if he realized I lied, that could be bad for both of us. Also, who knows what else he does to you.
Remember Arikada's henchman, and how she put an exploding bioseal in his head? Sure, he's so powerful he literally doesn't need to think about you as a threat, but it's still basic ninja dilligence to take what power and control you can."
Noburi: "All of that, yeah. I don't think he'll just screw you over and tell you to suck eggs or pay up, but he doesn't care enough about you to not take advantage of you if he gets the chance. Also, seals wear out over time, right? Not knowing anything about sealing, I'd still bet that whatever bioseal he puts on you wears out, since the whole thing Kagome always screams about when it comes to biosealing is that your body isn't a stable medium. And he's right, your body is changing all the time."