Only those "in" the mission now can see/hear the mission channel. Whats to stop "them" from doing the same with the 5 channel system you suggest?
I really wish they would build a ready room like AirWarrior had. The mission planner would open a room and would have control over who was allowed in (each player coming in would request entrance and a pop-up window would appear for the mission planner with the name of who wanted in. The mission planner would then decide to let him on or not and click to appropriate button on the window). This way only "known" players or players vouched for by "known" players were let into the mission briefing/mission. Not to mention the map that you could draw waypoints on that carried out into the game for assigned players and so on. It made setting up and running missions so much more "immersive" than what we have now.
I know we won't get "that" setup as it was done by some body else before, but something like it thought up by the super smart people <--- shot at brownie points.... at HTC along the same lines would be really cool.