The _biggest_ selling point with mumble (though I've not used it in anger in about a decade) was the way it tied in with the game you were playing and gave you in-game spacial audio.
Stand next to someone in game and they came through loud and clear. Move away from them sideway and it's now quieter and only through one headphone... I found it extremely novel and immersive when I used to play FPS games; everyone on mumble but absolutely not like being in a chatroom.