The tactic has its uses, mostly when advancing in fairly flat or rolling country where brush or trees obscured enemy defense lines. By constantly pumping out bullets, the entire formation would keep the enemy's heads down and could reach the point for a determined assault without becoming pinned down, since it was in effect constantly pinning down the enemy. In its purest form, Marching Fire was the 1944 equivalent of the traditional "push of pike".
The problem was, it didn't always work, and often the more traditional tactics of scouting forward and making a deliberate assault when the enemy was found produced the same results with fewer casualties. Marching fire gave up the advantage of shock. It placed more troops into the range of enemy weapons and denied them cover. When an enemy strongpoint was found, units could not concentrate to destroy it or maneuver around it, since the entire front was occupied by the advancing wave of infantry. Some units ended up on poor ground where they could not maintain the speed of the advance, forcing other units to slow down while under fire.--Stephen V Cole
One of General Patton's favorite tactics was known as "marching fire". It is often described in books about Patton, usually in terms indicating what a wonderful tactic it was. The general idea was a fairly dense skirmish line of infantry, with armored vehicles following closely behind them. The men and vehicles marched forward toward the assumed enemy defenses as artillery fired in support. Each man would, two or three times a minute, fire a round from his rifle in the general direction of the enemy (aimed if he had a target, into a likely spot in the brush if not). The soldiers were taught to shoot low (most soldiers tend to shoot high in brush or smoke) so that bullets would strike the enemy or ricochet off the ground and scream into the enemy positions as they tumbled in flight. The armored vehicles fired bursts of machinegun fire at likely points of resistance at odd intervals.