WHY KEEP JUNK? “Considering that the human body carries out ~10,000 billion cell divisions in a lifetime, the replication and maintenance of this viral junk heap requires a considerable amount of energy and resources. So why do we keep it around?”
http://schaechter.asmblog.org/schaechter/2014/06/retroviruses-the-placenta-and-the-genomic-junk-drawer.html