23andMe has a feature which allows you to check MHC compatibility. This is important for matching potential organ donors with those who need those organs. If a close family member is not a match (it’s a very polymorphic set of genes), then a co-ethnic is the next best bet. This is a major problem for those from ethnic minorities and of mixed-race. One day we’ll be able to “grow” organs from our own tissues, but until that day comes matching is still essential. Right now matching is done via drives and what not. But as more and more people get genotyped or sequenced, the information will be right there in the databases. Even racially mixed individuals with very rare combinations of alleles might find matches once the majority of the world’s population gets typed (though I haven’t done the math on this, has anyone?)!
On the other hand, imagine the ethical implications of an accidentally discovered unwilling match being pressured to donate.
From what I recall about half the people who end up being matched decline to be organ donors.* I think that expanding the pool of potential matches so that this situation is minimized far outweighs the cost of the risk Alex alludes to above.
* Donating is not trivial. Going to a drive to check for a tissue match is. So it shouldn’t be that surprising.