You would see the need for extra voip channels if you played TvT (PvP) like TacBF where there can be squads of 10+ players that can be divided up to 3 fireteams (FTs). Now imagine all those 10 players using the same group channel and what mess it creates. When the FTs could be using their own FT channel for FT related chat and only use Group channel to coordinate with Squad Leader.
This feature is desperately needed in PvP. Without it ppl will just abandon the built-in comms and move on to Acre/TFR etc. cos they provide this function (+ a lot of other voice comm related stuff that A3 is lacking or are broken/suck e.g. bad attenuation of 3D voip atm)