The fighter holds up at the lower levels because the majority of enemies are "fighter," "small fighter," and "fighter with rubber forehead." As soon as enemies become "fighter with wings" or "fighter with frickin' laser beams for eyes" or "fighter with bees in his mouth so when he yells he shoots bees" the regular fighter's usefulness diminishes.

And then we come to challenges that aren't fighters - disarming traps, breaking curses, stealth, and so on. The rest of the party will frequently need to expend their own resources (for example, an extra casting of fly or invisibility) just to shepherd the fighter through a situation that his "infinite spell" doesn't apply to.