>which is rather poor design for a competitive multiplayer game
Watching that video, I would guess the environment isn't so much competitive (even single player JC2 is very no-particular-goals-in-mind-just-blow-shit-up oriented) as it is mass chaos. It looked like what would happen if /b/ bought an island. So cheating is probably only a minor concern at this point.
While cheating may not be a problem in a noncompetitive sandbox environment, griefing is. Consider a dumb client that fakes logging in to the game server and sends thousands of explosions to a player's screen at once, or drops hundreds of jets or explosive barrels everywhere instantaneously. There is practically no limit to sending these hand crafted packets in comparison to how fast the GPU will be able to render the resulting actions. So now, you have a massive DoS problem on your hands. Ideally nobody will do this, but my point is lots of problems can come up when you trust the client.
DayZ is also not "competitive" at its core (and Arma2 even has anti-cheat measures built in -> BattleEye)... this doesnt stop script kiddies from server-hopping and killing/teleporting every one (even though they have no benefit from it beside the "lulz")..
Totally different game. DayZ is about solo play until you encounter other people, and even without cheating 90% of encounters with another person are shoot first, ask questions later.
Additionally, Just Cause is about just grabbing the nearest car and gun, then going out and causing chaos. DayZ requires players to spend literally days acquiring equipment, finding vehicles, fixing them. Finding a vehicle in DayZ can be a super rare find. In Just Cause 2, cars are everywhere.
Basically, cheating is a problem in DayZ because of the extreme scarcity of the resources. In Just Cause 2, there is no such scarcity. Therefore, cheating in DayZ ruins the game for everyone. Cheating in Just Cause 2 would probably go unnoticed, unless the cheater decided to become invincible.
Watching that video, I would guess the environment isn't so much competitive (even single player JC2 is very no-particular-goals-in-mind-just-blow-shit-up oriented) as it is mass chaos. It looked like what would happen if /b/ bought an island. So cheating is probably only a minor concern at this point.