Why not rank fire?
At least 3 good reasons:
1. As devastating as it may look, it only has a devastating effect on morale. Doesn't really makes any real damage. From the 160 shots fired by the three ranks, only about 20 casualties where inflicted, mainly from the enemy's first rank. This is due to the rapidity of the volleys. In about 5 seconds they unleash 160 bullets, and not always gives time for the hit casualty to fall
So he can be hit again and again. Makes perfect sense nowadays to shoot somebody with at least 10 bullets, just to make sure, maybe a grenade too , but back than a man would only get ten rounds per campaign, sometimes only three. So it was just a waste of ammo, not really a destroyer.
2. To allow the rear ranks to fire, the man in front have to kneel and start the reloading process, witch is not how you proceed with a cumbersome smooth-bore. These weapons had long barells compensate to it's low accuracy and where harder to reload in crouched position, so the already slow process of reloading got further longer. All this is fine because even so, a steady handed soldier could fire 3 per minute, but no sound minded CO wold allow his man to kneel in the middle of a firefight. The reason for this is obvious: we go back in high school. I don't know about you guys but I sprinted the 100m in under 13 seconds. That's faster than a reload time, and they would fire at each other from shorter distance. If the enemy starts a well timed charge, it can reach-Them, while the first rank is still wanking their rods, the second rank is aiming the bullet into the barrel, while the third is just bite open the cartridge and trying to pure some powder into the breech and the half cocked firing mechanism or lock. Now this is no way to receive an assault. Not to mention a cavalry attack! They would run circles around the company before they could reload.
So, no kneeling, no rank fire.
3. Besides the other flaws of this drill I will depict an in-game flaw, witch is also the flaw of other firing drills, except the volley. The fire sequence starts only when everybody is ready to fire. U can easily mess this up by advancing a unit to just enter their firing radius's extreme flank, this will trigger a response of a small group of 5-7 men to fire upon your unit. Quickly retreat and change them, or just start firing in them. Meanwhile those 5 or 7 will start the reloading process while their comrades wait patiently to be massacred.
That being said, I'm avoiding the use of this drill.