I'd imagine that facing a row of soldiers aiming things that go "boom" at you must have been an extremely effective way to break morale no matter how inaccurate the projectiles are.

Besides, all that hot lead had to go somewhere. Maybe you wouldn't be hit by the guy in front of you, but you'd still risk getting hit by the one behind him or the ones next to him. Arrows may have been more precise, but at least you could see where they were going.