Arguments for organ donors?
Science. Organ donation isn’t just for transplantation, they can also be used in research to help fight disease and further progress the human race. Donating your entire body as a cadaver gives medical training to students and the organs can be used to research any ailments you had.
Ensure you are dead. One eccentric reason for organ donation is to prevent being buried when you are not really dead. There are many cases of people suffering from something like a diabetic coma where they are pronounced dead but really aren’t. Some of these unfortunate people have woken up in coffins days later (evidence: claw marks in side upon exhumation). If you donate your organs there is a guarantee you will be dead and not wake up buried alive.
Family. A common occurrence when an organ donor dies is to challenge the authority you gave for your organs to be donated because they done want their loved one to be ‘harmed’. A lot of the times the ruling falls in favour with the family and the organ donation policy falls moot.