Believability and authenticity may be something, but who said it's required. Elements may be dropped with the theme intact if it's effectively replaced or if it doesn't even have that much influence on the atmosphere in the first place. Like I explained before, the atmosphere and the plight of both sides are explained much through the environment.
On top of this, the setting is an action-based multiplayer thriller video game that has full liberties. Why base it on tool-istic things like overused swearing and unnecessary brutality. In fact, it would actually make the sides less likeable, and would actually make the sides less grey. Also swearing and brutality bring out toolisitc qualities depending on how it's used. I shouldn't hear a swear every minute unless there is something bad happening or if it over the top.
One of the reasons why COD puts me off a lot is that they use this overaggressive frat boy jock archetype that's not just displayed in many of the people that play multiplayer, but is actually displayed in-game and it's media. Things like the homophobic controversy, including that terrible video about grenade spam, the overly violent attitudes of the Spec Op team, use of Eminem, the call for revenge that Cpl. Dunn stated, and the fact they used No Russian as an anolog as a spark for war when literally thousands of other things could have been chosen. It's based on realistic qualities of real soldiers, but they had liberties and they weren't trying to tell a story like Generation Kill. Call of Duty has been always honorific historical fiction (using real world element to turn something into a ballad) and they just gave up around MW2. I would have had the soldiers be mostly honorific while having some believable viewpoints and attitudes, which was mostly the case when war was going on, but then when there was a pause, reality struck and they were the army tool stereotype that I can't stand.
Nowhere did I state that I like COD (I hate it with a passion) or that more mature language should've been implemented with Brink. I'm fine with the T rating of Brink.
I'm just arguing against the overall statement that "violence, language, six, etc." shouldn't be in video games at all, implying it is a tool of developers to make people think, "Aww, man, they say the f-word in this game, I'm totally gonna get it," or "man, my character can have six in this game. Definite buy for me."
Some people may buy video games based on these credentials, but the majority do not. The majority buys video games on a much more important fundamental, or rather, two; gameplay and story. Splatterhouse based itself around one thing - gore and violence. However, the gameplay was crap, so it didn't sell very well and was critiqued throughly.
However, these factors (language, violence, six) can build and improve upon the story/gameplay of video games when implemented correctly.
Mass Effect and Dragon Age have six, but it fits with the story and the gameplay, since it is an RPG and adds to the overlying story of each.
Homefront uses violence to show the atrocities humans can do to each other, showing mistreatment of Americans by Koreans as well as the abuse of Koreans by Americans. This helps add to the story, showing that evil exists on both sides of a conflict, instead of the Call of Duty approach, where the only evil-doers are the Russians while the Americans are squeaky clean and can do no wrong.
Duke Nukem and Bulletstorm use excessive language, more than any human being would ever use, and have made those video games what they are. Bulletstorm is about getting points for killing enemies in new and different ways, which cannot be approached with a serious/realistic video game. The language fits it, using over-the-top swearing to complement its over-the-top shooting. And Duke Nukem...is Duke Nukem. Duke Nukem 3D is an amazing Doom-clone that embraced and portrayed the stereotypical macho man, here to kick ass and chew bubblegum. However, there is no bubblegum available.
Gameplay and story are the main reasons people buy and play video games. If graphics and sound are a little lacking, these can be overlooked for a solid base in these foundations. And if six/language/violence fits with the gameplay and story, its perfectly acceptable with these to be implemented.