I can appreciate the fact that organs are needed but you can't say to people that YOU decide if they get to keep all their body parts, that is their choice. When I went in for my driver's test, I went 2 days in a row, both times the lady (SAME lady) asked if I wanted to be a donor, I said no both times.
Besides, that kind of thing would make me nervous because say you were in an accident, and were hospitalized, and there was a chance that you would recover but the doctor opted for an early pulling of the plug so he can cut you up and ship you to other hospitals; my mom's best friend's granddaughter was in that situation, the doctors lied and said she had no chance to recover so they could harvest her organs, and when the autopsy came back her parents found out she had a very treatable problem, but the doctors said she was brain dead and there was no hope so they could get their hands on her organs.