You may have noticed that I’ve made something of a habit of flouting prescription and unapologetically using they as a non-specific third person singular pronoun — along the lines of “he or she”. As any good, rage-filled prescriptivist will gladly inform you, this is WRONG WRONG WRONG.
But it’s not, as Geoff Pullum insightfully argues. In summary: singular they is historically attested (by good writers, even), is hardly semantically or syntactically illogical, and is clearly useful. There is no conceivable harm that comes from using singular they. But I suggest reading his post, because it’s an excuse to bask in the glow that is Geoff Pullum’s writing style.