Why wouldn't we need microphones for Live?
I mean, in Modern Warfare 2, it's not the game or XBL's fault that it's always a bunch of whiny bastards that cuss you out and whine whenever they lose or get killed, it's the players' fault.
Games like Burnout Paradise and Team Fortress 2 can pretty much require voice communication though. Burnout, you do co-op challenges and have to plan pretty much everything with everybody. TF2, obviously, it's a good idea to tell people that there's an enemy sentry gun in your basement or a medic-heavy combo flanking you.