To be honest, the allied AI isn't meant to be good. Think of it this way: It wouldn't make any sense for you to be the only one fighting on your side during the games, i mean, it is the entire planet under attack. So it was mandatory to add human AI to the game. Secondly, you play the role of a multi-million dollar super soldier, so the friendlies aren't going to be as skilled at combating the enemy as you. But they aren't meant to fight your battles either. So the only true purpose is to give the game a slight sense of realism. In halo: ce, you were fighting on a small ringworld with almost no allied support on the planet. But in halo 2 and 3, the ai is fighting on their home planet. this makes it mandatory for them to be abundant. if bungie wanted the game to be like a squad combat game, they could very well improve the allied ai to be similar in skill to your buddies in rainbow six and ghost recon.