I understand your viewpoint, and in some cases it would apply to me. However, we differ in what we think is important in a game. I personally like to hear the grunt of Charr, or the clang of swords over a hill not visible to me, or even my own subtle footsteps on grass, dirt, wood, or metal.
I know, I may be wierd and in the minority--but I like ALL my ambient sounds. MP in a no-brainer, you HAVE to hear every little noise. But the crazy thing is, there are people that run around playing CS and BF2 with music on, or even worse, NO SOUND! People are bad enough FPS players, they certainly don't need to drop another 45% efficiency because they are deaf.
I suppose the first step to getting good in a MP game is to have a competetive streak in you. So that when you get owned from behind for the 10th time because you didn't hear homeslice enemy dude open the door behind you, you will get tired of this, stop playing music, and try and become a better, more efficient soldier.
But hey, I'm a FPS fanatic...play how you will, enjoy yourself, but be warned, I am the guy who knifes you from behind because you crank the volume on the chorus to "Hit Me Baby One More Time!"