A friend of mine and I were playing Halo 2 years ago, and we were goofing off in one of the Arbiter levels on Heroic. Neither of us were particularly pro at gaming, so it was a challenge for us. But somehow, despite the difficulty level, a single Hunter ally NPC stayed alive as far as he could follow and even saved my stoic alien butt twice. Not that there's much of a consequence to dying in Halo co-op, but it was downright surprising to see a giant green explosion take out the bad guy lunging for you.
And if you aren't familiar with Halo NPCs, rarely do more than two or three of the misc ally NPCs make it to the end of the level on Normal difficulty unless you consciously babysit them. On heroic? They drop like flies.