You know, you're right.
And I guess, using your logic, it would be OK to take comatose people who are going to die anyway and use them for experiments for new techniques on curing diseases.
Comatose people aren't suffering in the slightest.
With their consent yes, because in the case of comatose people who were old enough to comprehend the matter, they'd have had the opportunity to give consent. For anyone who never reached that stage (embryo, anencephalic, profoundly retarded, etc.), I would delegate the decision to the parents. Personally, if I were comatose and clearly going to die without regaining consciousness, I would want to either donate my organs to help other people live, or let my body be used for research. It wouldn't hurt me a bit, and would be selfish not to.