Interbreeding is vital to the continuing vigor of a breed. There can occasionally be undesirable offspring born with congenital defects and deformities when weak genes reinforce each other to become prevalent, but there also occurs the very hybrid vigor that Popenoe et al discounted. Science shows that humans and Neanderthals interbred, giving rise to humanity as it currently exists (Sankarararaman, Patterson, Li, Paabo, & Reich, 2012). If interbreeding had not happened, then it is possible that humanity would have never reached its current level of development.
Throughout history, there have been prime examples that serve as advisories against maintaining genetic homogeneity. The royal line of England during Queen Victoria's reign was afflicted with hemophilia, mainly due to a lack of genetic diversity in the royalty of the era.