FITNESS BENEFIT. “Another potential benefit of cell–cell fusion is hybrid fitness: If, as seems likely, there existed a variety of fledgling protoeukaryotes in the population that eventually gave rise to the last eukaryotic common ancestor, then their fusion would be expected to have yielded more gene-rich and occasionally more successful lineages.”
http://cshperspectives.cshlp.org/content/6/3/a016154.full