Del Rio worked wonderfully in Halo 4's context. Captured perfectly someone unsuited for command.
It does raise, in the context of the game, the question "why the fuck", but in larger expanded universe context it works wonderfully.
A weird kind of character for a video game, perhaps, but one reason i really like the character.
A problem i have with Cortana in Halo 4 is that she doesn't feel like an AI but a human character. OK, Halo 3 shares this issue. Before, Cortana may be human-like but ultimately she doesn't feel quite human. Yet she is supposed to be an AI. Feels like narrative conflict. Ironically, i guess this is a whole point in the game, that she is a human and the Chief a machine, which is pretty good examination of those two, but IMO it clashes with previous depiction of things along with how i think AIs should be portrayed, which is as "not human".
This is not an issue limited to Halo. Too many of scifi treats AIs just as effectively humans, perhaps superpowered in a sense of having super intellect or super reflexes, but human nonetheless.
AI, just like aliens, are not human. Hell, aliens are usually more human, sharing similar physical requirements and needs. But AIs? They function in different medium, so to speak, with very different needs. They may have all the human knowledge crammed in them, but knowledge doesn't make humans human (if anything, cramming a lot/too much of data into human brain likely turns it into something else).
OK, fiction portraying alien aliens isn't too common, so it is pretty hard to do that with an AI. But at very least i wish more tried that.
Really liked Superintended in ODST and Auntie Dot in Reach as they felt like AIs. The former was especially nicely portrayed by limiting its interaction to using environment it controls to "talk".