Bergens aren't sexy - that's a lesson to be learned early and often. Your bergen is your bedroom, kitchen, wardrobe and storage loft.
It does wonderful things when you get a second to brew up, or kip down or even just rub an alcohol wipe over your face (anyone who's ever been devoured by biting midges will know this sublime pleasure).
But you don't wear it in contact, most large loadbearing packs are now designed specifically to fit a daysack under the top flap. So if the worst happens and you get hit up while wearing your trundlebag you drop it, grab your daybag and carry on.
For that reason it doesn't really make sense for ArmA2 units to carry their main packs unless you plan on lots of character building walking from A to B in your missions.