How important is the infantrymans rifle? Thats hard to say, because of the long sequence of events that must happen before he can ever use it (to decisive effect). Marksmanship skills, small unit tactics, and combined arms practice must be mastered in training before they can be employed on the battlefield.
When soldiers do arrive in-theatre, howitzers, mortars, machine guns, and other crew served weapons will do the bulk of the shooting. The infantryman and his rifle shall play their part, adding their own modest firepower to the battle, but it will be to marginal effect -marginal in comparison to the real killers out there, which are tube launched artillery shells- and only with the aim of suppressing the OPFOR and fixing him into place. But once that happens, the foot slogger can work his way closer and closer to the enemy, acquiring a position of advantage where he can reach out and touch them with impunity (or open up an opportunity for his buddys to do so). And although a soldier may only ever kill a handful of enemys during a battle, or even a whole tour, the effects of his actions are multiplied many fold when echoed by his comrades: Enough, perhaps, to convince the enemy collective to withdraw their participation, and forfeit the battle. Successful battles open up opportunitys for theatre level exploitations.
So, in summary, the infantryman and his weapon are the integer exponent for which all wars are won or lost. Despite however much attention the larger factors in warfare garnish, they are all structured upon the building block of the humble foot soldier. Let that never be in doubt.