I disagree Singular theytheir has been used by educated people in a formal register for a long time In fact I have a copy of a British passport application that uses it in that sense I prefer it sometimes and intend to keep using it Grammatical rules are fluid and prescriptivists who think they are not can have a stifling effect on discussions and even distort peoples perceptions of some groups within a society based on some perceived error in the way they speak For an encyclopaedic style all that matters is whether a significant number of educated native speakers would find a particular usage acceptable Proscriptions on split infinitives terminating sentences with preposition and other poor grammar are for another age