THEORY FOR THREE HOSTS. “For such life cycles, we propose here that maintaining a second intermediate host in the life cycle can be advantageous for the individual parasite to increase the intermixture of different clones and therefore decrease the risk of matings between genetically identical individuals in the definitive host.” An elaborate way to avoid the prohibited inbreeding.
http://onlinelibrary.wiley.com/doi/10.1111/j.1420-9101.2005.00895.x/full