Jim Baumann, who writes the "Grammar Moses" column for the Daily Herald in suburban Chicago, recently held forth on "Why 'they' is not a singular pronoun":
"When you use a singular pronoun, it lets the reader know you're talking about one person. When you use 'they' across the board, your audience can get confused about how many people you're talking about — as well as whether you are talking about someone who has run-of-the-mill gender identification or something different. If that matters."
I don't know anyone who uses they across the board. I do know people who use they when the antecedent is an indefinite pronoun: anyone, anybody, everyone, everybody, someone, somebody, no one, or nobody. I also know that that usage has been current in English for centuries and is listed as a standard variant in the original edition of the Oxford English Dictionary.