Malbim: He showed that the Shevatim are united like one body, and just like there cannot be life and astuteness in a dissected body, for the limbs are dead, so a spirit of life and Shechinah cannot take effect on the entire nation if its parts are separated and the Shevatim do not unite like the limbs unite to enable life. And just like if one limb rots, it makes the entire body ill, and doctors try to cut off the limb lest the entire body perish, so here, if a small number of people are lost (spiritually, others) must strive to make them repent or eradicate them from their midst. Surely, along with the limbs he sent the story behind them.