Muting is absolutely necessary. Play any online video game for 5 minutes and you'll understand it's a necessity.
As a general concept I agree, but this is limited to your facebook friends. This isn't randomly joining a multiplayer server, this is intentionally adding friends to a space. Ostensibly, you should know them. Adding muting capabilities seems like some weird power play that'll just lead to bullying. If you don't want someone there, just remove them. Hell, unfriend them.
And you've been able to mute people in various group voice chat programs (made for talking with friends) since forever.
But muting your own friends while gaming with them? Nah.