From my perspective, the only difference between an AI and a 14-year-old American is that the AI is polite.
I think this is what impressed me about Halo Wars. This is a game, like all of the Halo titles, that is clearly built for multiplayer. Around half of the game's achievements are multiplayer-only achievements. And yet, when I firmly informed the game that the "multi" part of the phrase "multiplayer" would be performed entirely by the game's artificial intelligence, it shrugged, started the game, and began duly handing out achievements.
I prefer playing bots. I really do. Not just because on average the XBox Live community is a cesspit of pre-pubescent homophobia and racism which Halo enthusiastically scrapes the bottom of, but because bots offer a more meaningful experience. They play at a level of skill determined by me and they make decisions based on fixed rules, which means that I learn faster by dint of being able to try different things in similar situations, and I can accurately judge my skill gain as while I am getting better they are verifiably remaining the same.
When I say that I want to play multiplayer, sometimes I genuinely mean I want to play with other humans. This is the case with most every co-operative game, and also with rare competitive exceptions like Soul Calibur and Call of Duty 4. But mostly it means that I want to experience the multiplayer content. I want to capture a flag; I want to get a killstreak; I want to narrowly win a hotly contested deathmatch. Experiencing the content is different from experiencing the competition. I don't need to feel like I won teh internetz. I don't need to beat any other real people. I could be the world's worst player and still enjoy beating some bots.
Requiring you to be beat real people in order to experience the thrill of beating people is like a driving game that puts you in control of a real car in a real race against real drivers. It's certainly an experience but to a large extent it's redundant. The very attraction of games is their ability to simulate scenarios and experiences that may otherwise be too difficult, improbable or outright impossible for us to enjoy in real life. Defeating Sephrioth in Final Fantasy VII is not any less enjoyable for the knowledge that Sephiroth is not the avatar of a Minnesota high school student.
It's particularly an issue when it can be hard to identify an AI. In Halo Wars, if you have the chat turned off, the only way you could tell a human player from their gameplay is when they make a mistake. The only signs of human intellect to be seen are indecision, ignorance, and poor judgement; without them it's anyone's guess. AIs here are functionally interchangeable at all but the highest and lowest levels of play.
So in short I had a blast with the multiplayer component of Halo Wars. It was exceptional. It turns out that the average denizen of the gaming internet can indeed be replaced by a machine to the benefit of the world in general. For perfect realism I suppose there should have been an "AI will randomly quit mid-match" check box, to be toggled on or off, but in general I can't help but think that an MMO where all the other players were AIs might be something of a hit.