Ingame VOIP is in principle a good idea, but in practice people prefer TeamSpeak or Ventrilo. I think an excellent opportunity was missed here to use ingame VOIP with a unique functionality.
I think ingame ArmA VOIP could have, as well as it's current channels, the possibility to place the voice with the player's ingame character. So that if you're close by you can hear him fine, but the further away you are the fainter he is, and even be able to locate him via direction of sound etc.
That way small separate groups could communicate between themselves without everyone hearing their plans, people could inspect suspicious sounds of conversation, stuff like that.
I think that would be a game first if I'm not mistaken, and a biiiiiig plus for immersion.