H4 should definitely have smart allied AI. A squad of Marines should be able to hold their ground against a squad of Grunts and Jackals on Heroic. Allies that could dodge grenades, shoot accurately, and drive well will definitely be welcomed by me.
But by no means should they hold your hand throughout the entire campaign either. To have smart allies and yet not have them be too powerful is simple: we decrease the damage that the allies do to the enemy (besides with certain weapons like the Warthog gun). This way, we can have smart allies that can drive and shoot well while letting us fight our way through unhindered.