It's not so much the 1 on 1 scenario that's bad tactics. Sure, one guy could shoot at it - but it can shoot back, and at that point you're about even.
The problem is when you start worrying about large numbers. In a fight, essentially only the front rank of humans can shoot at each other without endangering their allies. As soon as you bring in something tall on one side, the opposing side can now bring fire to bear from all the humans behind the front rank, by shooting over their heads at the tall target. Unless the tall target can match the durability and firepower of the rear ranks on the opposing side, it's toast in a fast way.
Admittedly, indirect fire weapons like mortars, grenade launchers, artillery, etc, widens the depth that you can engage, but so does terrain. Air support, guided missiles, and teams trained for action behind enemy lines widen this even more.
Still, one of the basic tenets of combat tactics is to maximize the number of people on your side that can engage the enemy while at the same time minimizing the number of people on the opposing side that can shoot back.