1. The majority of players are right handed and, if they are in a covered position, their left arm is behind cover, invisible to those coming towards them from the front while, left-handers (like myself), will be much more easier to spot (less or none of our armband will be behind cover).
The reason for this is so that players only have to look for one arm. Otherwise you see their left arm and say...well, I don't see my color on that arm and I have a good shot at them...but I need to wait until I can see the OTHER arm TOO before I can be sure. They wait to see the other arm and confirm that the player is an opponent, but by that time they have missed the shot. If they know what arm they need to look at confirming friend or foe is much more efficient than not knowing which arm to check. If you KNOW that your teammates are taped on the left arm, and you can SEE an untaped left arm, you're done. SHOOT!
If I am commanding a scenario event that requires everyone to be taped I always ask that all players on my side tape the same arm for the same reason.
Yeah I can see that but, why do most fields that care to tape a specific arm, pick the left arm? cause most people are right handed. So, in my opinion, remove the advantage of right-handedness by putting tape on both arms.