We see over and over how the real effect of ta'veren is always balanced, how the highly improbable bad things are countered by equally improbable good, with miraculous survival balanced by freak fatalities and so on. Even Mat, whose ta'veren effect seems to translate into a lot of good luck, is balanced by finding himself in incredibly unlucky scenarios, and drawn to times and places of danger. So what would be the point of such random swings if every good event is balanced by a tragic one? Because the Dark One's power is so far beyond any human strength or capability (it is stated in the books and by RJ in different ways to the general effect that there isn't enough power available to humanity to beat the Dark One directly), that only by the wildest, once-in-a-million chance, can mortal mankind hope to beat him. Basically, the function of the ta'veren is to make that pendulum swing wider and wider, with correspondingly greater highs and lows, like a pumping up and down on a swingset, until the right peak of good luck is achieved, and Rand can win.