Why did HaShem create twins (Yaakov and Esuv) if he could just create one righteous person who would be Yaakov with the good traits of Esuv? I know Esuv was supposed to support Yaakov, but Yaakov didn't need support so long as he followed the path of HaShem. We see that even when Esuv was hunting him, he somehow became rich with food and gold.
Another Kabbalistic idea is that Eisav and Yaakov represent, respectively, the spiritual worlds of Tohu ("chaos") and Tikkun ("correction"). A presentation of this idea is here. A couple of key paragraphs:
Esau is the raw, untamed energy of Tohu. He is a destructive force, because he lacks the discipline and control that would channel this energy in a useful, constructive way. But he is also a very powerful force -- far more powerful than the constricted and defined energies that animate Jacob's correct and orderly world. The challenge, as we said, is to bring together the cosmic twins in a way that exploits the best of both worlds: to marry the immense energy of Tohu with the focus and control of Tikkun.
So the quest to unite Tohu and Tikkun extends beyond their lifetimes, to the nations of Israel and Edom. The eight kings which "reigned in Edom, before there reigned any king over the children of Israel," are the volatile forces of Tohu, while the people of Israel proceed to Sinai where they are entrusted with the 613 commandments that serve as the vessels for tikkun olam, the correction and civilization of the world. The conflict rages on in the battles between Judah and Rome, between spirit and matter, between law and lust, to be resolved only when the struggles of humanity culminate in the day when "the saviors shall ascend Mount Zion to judge the mountain of Esau."
The Zohar in Breishis says the reason all the avos until Yaacov had one good and one bad child is because after the sin in Gan Eden of eating from the tree of knowledge, the zuhamah (spiritual taint) was still in them. So in order to remove it there had to be a bad child for each until Yaacov who had all 12 good children.
I always felt the reason the Avos had Yishmael, Aisav, brothers fighting, story of Dinah, etc. was to give us Chizuk. When a parent has a child that is at risk, children fighting or not talking to each other, or any other tragedy it gives them Chizuk when they can look back at the Avos HaKedoshim and see it happens even to the best parents.