I dismiss too much as 'dramatic tension' I feel. That's why I rely on people like J Esterline to point it out for me!
J Esterline (email@example.com) writes:
A quick thought: Why doesn't Gravemind intervene on the side of the Arbiter in the fight against Tartarus? After all, there is evidence of a fight for the Index earlier on. Is the outcome (the Arbiter stopping the activation of the Halos) a foregone conclusion? One would think not, given how close the Halo is to firing at the end of the fight. Maybe Gravemind's omnipresence (at least through infection forms, as commander of the Flood), or his near-omniscience, is showing a deficiency here; or, perhaps, his willingness and confidence to gamble in such a situation is exhibited? Surely he is a "sentient being" and thus susceptible to the destructive power of halo; even if being on a halo is protection (this is never stated, to my knowledge), his flood would be starved for hosts, as intended by the Forerunners. All in all, a bit of a puzzle, it seems.