I understand that Minfilia isn't a fighter, but what does she do? (And what is that knife for?)
Manage the Scions? We already have Tataru as secretary, and Minfilia's "management" usually boils down to "dispatch the local Scion and the Warrior of Light to resolve a crisis."
Yes, you could argue that Thancred's possession was his own fault, but if Minfilia really was a proper manager, she'd know how to balance the needs of others against the needs of her people. Thancred overworked himself to help fill Louisoix's shoes, but as the manager Minfilia is responsible for looking after everyone and ensuring they're fit to go on missions. If they're overworked to exhaustion, they're liable to half-ass it or be killed in action, and then nobody else can be saved. Being a hero isn't all sugar and rainbows. Even if it's not a pleasant part of the job, sometimes sacrifices have to be made. You can't save everyone.
We didn't really need Minfilia to figure out how the Echo works, and to be honest we still understand only the barest of details about it. The Ascians do all kinds of things with the Echo we never dreamed possible (they're able to essentially break the rules of reality with it), so to say she "figured out the nature of the Echo" is giving her way too much credit.
Research? Ehh, I guess. Still, I thought that was more Urianger's job (and he's no slouch in combat, either). R&D is mostly Urianger and Moenbryda, respectively. Or was.
Are her managerial skills useful? Fo 'sho. ... but we never see her do any actual managing. Every time we go talk to her she's just standing behind her desk in the Solar. If she were really out there supervising research and such, she should be... you know... outside her room.
Not saying she should die or that I want to kill her, but I do not see the point in having 2.x Minfilia around (except for fanservice, but that's not a valid reason to me). I'd save her given the chance, and would be very happy if she came back in a more useful fashion at some point, but aside from exposition and as a figurehead I never felt 2.x Minfilia was all that important.
It strictly doesn't, but the thing with the knife is that it's a Main Scenario Quest. Everyone has to do it at some point, so it is a 100% undeniable canon fact that Minfilia is armed, yet she does nothing to save her own damn self.
... rankles just a bit.