I'm going to suggest that cancerous killer T cells give an evolutionary advantage to the genes inside an organism that has it. How could getting cancer be an evolutionary advantage to a gene you say? Well if an organism lives too long then evolution and hence adaptation within a species is reduced or slowed down. Even if you don't accept evolution at the organism or species level it would still make sense that an organism that lives too long, especially beyond the age it can procreate, would be a disadvantage. One of the challenges with thinking about evolution however is the ease with which we can come up with just-so stories.
How is the size of organisms related to their lifespan? Smaller organisms usually have faster metabolisms and hence shorter lifespans but is there somewhat of a universal scale of this? Large organisms that live too short wouldn't be very evolutionary advantageous... Small organisms that live in extremely variable environments that lived for long periods would also seem to me to be an evolutionary disadvantage. What do you think? Cancer aids the survival of genes? The selfish gene...or cancer our saviour?