Sean Carroll in Preposterous Universe:
The announcement we wait for every year has finally come in, and the American Dialect society has chosen their Word of the Year! That word is: “they”. It beat out other finalists such as “ammosexual.”
You might think that dubbing “they” as the Word of the Year is some sort of lifetime-achievement award, since the plucky pronoun has been part of English for quite a long time. But the prize has been given, not for the word itself, but for a particular usage that has been gaining ground for a while now: the singular“they.” We most commonly use the word to stand for the plural: “Jack and Jill went up the hill, but once there they realized they had forgotten their pail.” More and more, however, we’re seeing it used to denote one person at a time, when their sex is unknown to us: “The robber left no fingerprints, but they did leave a note to taunt the police.”
It would be somewhat more traditional, in some circumstances, to say “he or she did leave a note.” It’s a bit cumbersome, however, and to be honest, the real tradition is simply to act like women don’t exist, and say “he did leave a note.” The rise of “he or she” has reflected our gradual progress in remembering that human beings come in both male and female varieties, and our language should reflect that. (We can also try to make it reflect the full diversity of sex and gender roles, but while that’s an admirable goal, it might not be realistic in practice.)
Using “they” instead of “he or she” or just “he” is a very nice compromise. It sounds good, and it’s a word we’re already familiar with.