I'm going to speechify some in the IC thread too but just to get this out of the way-

Luceti risked the entire village to save Bil back when he was dying. Luceti also has tried time and again to work with the Malnosso, and for the most part the only one who's cooperated all that much has been Bil himself. She doesn't see any reason to not continue with that faith now, even if you don't actually "trust" him or whatever. A lot of the other enclosures that think they're sympathizers, and playing it safe has only cemented that more and more.

Haruhi's vision of Luceti's future involves uniting people from across enclosures and making a big blow like this will help with convincing a lot of the resistant ones they're legit (or so she assumes) and frankly unless they sabotage the plan (and why would they? She's not a combatant herself, but the Third Party has tried to kill her before and never done anything kind to her and her's) they're going to do it anyway and the Malnosso will be displeased.

So they might as well throw in with the people who stand a chance of actually being their allies (for once)