I do use the HQ AI, both in single player and, to a lesser extent, in multiplayer. Most singleplayer games I play these days are mostly to try out build strategies for a multiplayer game, so I am not really much concerned about micromanaging my units or doing the best I can, I just want to see relative effectiveness versus other strategies, and the Theatre AIs are fine for that. In multiplayer, I find it quite hard to manage large fronts by myself, so I often turn over some areas to the AI. I've found that the AI is rather good at making full use of a front, both in terms of length and unit readiness, whereas players tend to order attacks in spurts and clumps - so the AI can sometimes help keep the micromanaging-style opponent so busy that he might make a mistake or not notice some crucial development (though we do have house rules to announce things like amphibious or airborne landings, especially when they create new fronts). It does require sufficient forces, though, the AI does rather poorly if it's outnumbered.