Earlier quoted context omitted.
Do you not get a "+1" for anyone of your choosing at any point in time if you donated to a stranger in the past?
How could they guarantee that? What if you need to call that right in and they can't find anyone compatible?
Compare to kidney exchange (http://transplants.ucla.edu/site.cfm?id=112): if your friend or spouse needs one, but you're incompatible, you can find another pair who has the inverse compatibility problem and swap with them. Or more, and form a chain (e.g. this 35-kidney chain: http://abc27.com/2015/03/30/paired-exchange-record-70-people...).
What I was referring to is Good Samaritan donation:
Many Good Samaritan donors choose to start chains because it is a way to help more than one person suffering from kidney failure. One chain typically facilitates anywhere from 2 to 30 transplants. The NKR pays for donation insurance for all Good Samaritan donor who start chains through NKR.
http://www.kidneyregistry.org/living_donors.php?cookie=1
EDIT: Ah, found it---they're called Kidney Vouchers :) http://transplants.ucla.edu/site.cfm?id=406 great name. Although limited to one predetermined person, so not exactly a blanket "+1".