Drug development is delayed due to a lack of human test subjects. Before a drug can be tested on people it has to go through over a decade of tests on animals. Even after all these animal tests things can still go wrong. Due to differences in animal and human physiology these animal experiments are not particularly effective but they are the best we currently have.
If I get hit by a bus tomorrow I will likely be put on a ventilator and at some point later they will unplug me and take my organs and put them in various other people in a fairly cannibalistic process. This happens because it is judged that I am no longer alive as a conscious entity and that unable to carry out basic functions, like breathing, for myself I lose the rights that come along with human life.
So if it is ok to harvest coma victims for organs that might save up to a dozen lives why not test drugs on them that could save millions? What is morally different in these two conversations
Doctor: I am afraid David is dead.
A) We would like to put his liver into an alcoholic
B) We would like to test AIDS vaccines on him for the next 6 months
People donate their body to science all the time. I was once at a funeral of someone who had died two years earlier. So why does your body have to be not breathing when you donate it?