Considering that most of the other civs span several hundreds if not thousands of years into the past, I still don't see why the American civilization has to be in every version of Civilization - apart from the obvious influence on the game's market. The Americans have been on their continent for only a few hundred years and before that they were Europeans, well, or Africans, Asians, etc.
I always rationalized that, well, the American civilization must include the Native Americans too, but when the first Native American civ appeared in Civ that logic went through the window...
Sure, USA is the only superpower (until the Chinese and the Indians get their economy together at least) at the moment, but considering how ancient some of these civilizations are, a few hundred years of independence and a few decades of hegemony doesn't really create a "civ" in my eyes.
Mind you, I'm not anti-American or anything, but to me the Americans stand out like a cyborg in the midst of an army of spearmen. That's how I've always felt since Civ I, and I apologize if I upset anybody with my views.
Ok, I think it's time for me to duck from all the incinerators, because I can hear the flames a-brewing.
The Americans are just as worthy as other Civs. Just because they are modern doesn't mean they should get left out.