Any HLA allele which becomes a more resistant allele would not simply become an inherent allele in all individuals. Through evolution, there will always be some pathogens that can become resistant to this allele, and spread to create a selection against the allele. HLA-dissortative mating can be considered a method to cause the adaptations that pathogens have to their host to become obsolete in their offspring; In other words, allow us to keep up in the "Red Queen's race".