The Mole That Poked His or Her Head Above Ground

All of my co-workers consistently misuse the genderless plural pronoun in place of a singular pronoun, as in the following example:

Someone left their book on the table.

Their is a pronoun used for more than one person (They left their books on the table), but here, only one person left a book. You wouldn't say Cinderella left their slipper at the ball because that doesn't make sense either.

People talk like this everywhere, and it's a reality I've grown to accept, like the prevalence of pro football or the existence of Crocs. Today, however, I found this in a kindergarten newsletter:

Every group of moles has a guard. The guard mole pokes his or her head above ground and warns the other moles of danger.

I was aghast at such an awkward, though grammatically-correct sentence from one of my co-workers. His or her head? What had caused such a pedantic shift?

I've come up with two explanations, possibly related:

1. Because the subject in question was the guard, the writer felt more inclined to use singular pronouns, as the is generally only used for one of something.
2. Because the sentence is about an animal, misusing their in place of singular pronouns seemed less appropriate.

Whereas in previous centuries, the teacher's sentence would have been commonplace, our language is constantly changing (and whoever be he who thinketh othewife might best have his head cleft clean from his body), and the singular their may be here to stay. I predict that in a generation or two, the casual their will be formally regarded as correct, and blog entries like this one will seem as dated as they are condescending.

Want to learn more?
The Genderless Pronoun: 150 Years Later, Still an Epic Fail
The Epicene Pronouns: A Chronology of the Word that Failed