I always have a mic on, but I am in party chat 90% of the time.
I'm either playing single player or playing MP with people I am in chat with.
If I am not in a party and am playing MP I wear a mic but often have it muted so I can hear everyone else but they won't hear me unless I want them to.
For the most part it is just simply respect for team based games and the hope your team will have players who know how to call enemy locations, danger points, etc.
But that usually falls apart when you get the screaming 12 yr old playing a game way above his age group or all the people who think you have to play every game mode based on K/D ratio.
Shhhhh! Be very very quiet... I'm hunting UNOcorn
How many Houngans does it take to start a Zombie Apocalypse? And why would they do that?