In competition matches, many maps are Conquest Assault style, where one side attacks and one side defends.
Zeller had huge advantage, compared to minor ammo issue, in competition matches.
1. OHK to the chest or at least 75% HP gone.
In the beginning of the match, a squad leader runs as fast as he can to a flag by using LIGHT ARMOUR and spending all stamina. AFTER the stamina is gone, or slightly before if playing more conservatively, squad members spawn on him to group the squad as best as they can to provide concentrated firepower in an area.
If you are able to kill him before kill the squad leader before others spawn, you have made an enemy team waste a bunch of time establishing a footing, while your team is able to set up offensive/defensive position on a flag.
People don't use light armour in public matches much, but its rather used often in competition matches, as infantry combat is more often than you expect, due to vehicles melt away in concentrated engineer prowess. In an infantry combat, one of the main goal was to send a runner to draw enemy away from a flag that your team is attacking (if none of the enemy falls back, then your runner takes over the back flags). Being able to take out runners from distance, with higher probability is better than having to send more soldiers to stop the runner.
2. You can blow up enemy mines from distance. In a blitzkrieg style rush in the beginning, in a vehicle heavy maps, enemy engineer will ALWAYS toss mines. Not that our vehicle can survive bunch of mines + engineer weapons, but being able to remove as much as possible (in this case, engineers usually also wear light armour so that they can run to the mine laying position as fast as they can). This blitzkrieg is usually used with APC, which have ability for a passenger to launch out of the APC onto a flag, or some difficult to reach position. Because of this ability to destroy enemy mines, Zeller is one of the least selfish weapons a player can carry.
3. You are very likely to die before you extinguish your ammo reserve. Having two more shots in the magazine does not net you more kills in the long run, especially in smaller matches (which basically is all competition matches).
4. Its the only clear lens scope our of the three rifles. If you have your texture to medium or lower, Zeller is the worst rifle, as everything is tinted red, but when set at high, it is depicted properly (like you see in the picture) where only outer edges are tinted and centre is clear.
4. Its the ONLY SNIPER RIFLE ALLOWED in a competition mod. You have no choice. In the competition mod, Sniper Rifle = Zeller. So using Zeller all the time is just better to not lose the touch when in competition situation.