For a long time, they was used in both a singular
sense. It was only in the 18th century when grammarians attemped to change English from a commonly evolving language to a static and controlled language ala French
and their L'Académie Francaise
that its singular use became an issue. Basing their rules on Latin, they dictated a number of such rules - see split infinitive
for another example and why it wasn't a great idea.
Considering the singular use of they only violates convention and actually helps communication by providing a singular non-gendered pronoun that doesn't sound like a bad science fiction term, I'd say this is a rule meant to be broken. Language evolves, and there's no reason why the nature of this word can't change if enough people say it should change.