And to add to that, the Campaigns can be more challenging if you take under account the "Drop In" feature, since the AI is replaced by another Human being, and that is actually the best AI a game could have for battles.

There is no AI (yet) that can replace a Human, specially not for a game anyways. :)

So I think in a way CA found a clever way not only to leverage the Internet itself and Steam but to enhance the game itself in ways that would have never been possible to program in to it.