I never use mics and I'm pretty damn good at games like Left 4 Dead. Say a hunter pounces you...well, you don't need to scream "I'VE BEEN POOOOOOOOOOOOUNCED!" down your microphone, the game actually tells me you've been pounced, so I can save you. In fact, I've been on teams where everyone was using mics, where I saved more people more often than everyone combined. Using a mic doesn't make you a better team player on Left 4 Dead. Not even on expert. I can tell if you want to avoid going through the barn. I can tell if you want to hide in a corner. I don't need to be told there's a tank behind me, because the nice, helpful music and my own awareness do that for me.
If anything, it makes me a better player. Why do I seem to be the only person who can tell the 'Sn1P3r_G0d1234' has been pounced without a mic?