When you play a game, you do things based on how it will appear to the people you play against. You occupy a dark space because you know that you'll appear in a way that makes it difficult for the opposition to see you. You choose a certain uniform because it fits the surroundings. You choose whether or not to take a shot from distance with a quiet weapon based on how easily the person will be able to figure out where you're shooting from. These are the things that make the game more than just W and mouse1. You need to be able to how you appear in the game world. That flies out the window if your scout sounds like a high powered rifle, if your dark uniform is actually bright pink, or, less egregious but equally revealing, some kind of "cool" street camouflage.
Some skins are more beneficial than others, some models and sounds give you more of an advantage than others. None of them are part of the game, but rather alterations to the game, and as far as I can tell, every other modification to the game that may confer an unfair advantage is considered cheating, and a bannable offense. This simply would not be a problem to anyone if people played the game as it is, and I don't believe that anyone's desire to have a Tommy gun in place of the P90 supersedes the importance of preventing people from abusing the opportunity to gain an unfair advantage by modifying game files.
This is silly, Blackmage. They are not playing different games. One is merely playing the game better than the other. This philosophical "you're better so you're playing a different game" argument doesn't really have a place in a debate of whether or not to allow people to, not philosophically, but literally play the same game with different content.