They need to know whether they should try and revive you in a hospital, or just harvest the organs and move on.
I would be sad, but I wouldn't blame other people for refusing to donate their organs.
Overall, I can't argue that organ donorship shouldn't be opt-out (or even mandatory) for the good of society. There is no good reason not to have it that way. Yet, as utilitarian as I am, I cannot help but feel that I wouldn't want MY organs to go to someone upon my death. Irrational possessiveness.
I'll leave with this quote here: