This is a story to make people understand what would be the result of greed. Who was the rightful successor is not the question. Pandavas were the sons of Dharma Deva, Indra, Vayu and Ashwini Devas (not Pandus!) So they had no right to succession.
This is like the Congress party leaders and workers claiming that Ms. Sonia Gandhi has the right to become the PM of our Country by virtue of her marriage to a member of Nehru-Gandhi family!!!!!!!!!!!
