I'm listed as a donor all over the place- health cards and drivers licences in 2 different countries.
I'm really not going to need my bits and pieces when I'm gone, and I don't want to be kept alive on machines if I'm unable to function on any kind of meaningful level, or facing an epic ordeal to recover.
I've had depression for years, and while I'm relatively happy now, I'm not exactly living a hugely active nor meaningful existence, I know my cats would be looked after and get over the loss of their primary meowing target, and my relatives would be ok with time, and i really don't need any of that heroic measures crap.
I live in a country with government healthcare, but I have private insurance too. I don't feel paranoid that doctors are particularly likely to write me off eagerly before my time for wanting of a kidney, or whatever. That really sounds like silly bollocks to me. Prognoses change, doctors face prosecution and being struck off by ethics boards if they don't seek to 'first, do no harm'- safeguards are in place.
I doubt they'll stick my organs into other people here in Canada anyway, as they won't take my blood for donation (I'm a BSE risk, apparently, go me!) but maybe they'd offer them to other British ex-pats, or manage to lob them over the Atlantic super-quickly before they go mouldy, or use them to train medical students, or to develop or test new drugs, or something. Rather non-sentient me, than some poor bloody rabbit or beagle or monkey.