One: We need it.
Two: We use it.
Three: We understand it.
We need a gender-nonspecific third-person singular pronoun to ensure inclusive writing that isn’t awkward. Generic “he” just doesn’t cut it anymore; extended use of “he/she” and “his/her” in writing is clunky; random switching between “he” and “she” is distracting at best, confusing at worst. Additionally, having the option of “they” accommodates people who don’t identify as either “he” or “she.”
English speakers and writers don’t just use singular “they” now, we’ve been using it for centuries. We’re talking pillars of English literature here, too: Chaucer, Shakespeare, Austen, and so on. Even the King James Bible uses it (and frankly, if it’s good enough for God, shouldn’t it be good enough for the rest of us?).
That leaves the issue of clarity: If a usage choice introduces ambiguity or confusion, it’s generally not a good choice. But singular “they” is always clear (unlike singular “you,” which has led us to y’all, youse, yinz, you’uns, you lot, etc., to specify singular or plural – and which is also an example of a plural pronoun shifting into the singular, so unless you complain about singular “you” being ungrammatical, the “singular they is ungrammatical” argument holds no water). We understand when “they” refers to a group of people, and we understand when it refers to an unknown or unspecified singular person. Clarity is also the advantage “they” has over pronouns such as “ze,” “hir” and “em” (and dozens of other failed pronouns over the past 150-plus years): People know exactly what it means.
We all use singular “they” without even thinking and read right over it with total comprehension. The time has come for it to be considered standard.
Everything you ever wanted to know about singular “they” (The Stroppy Editor)
Choosing the Singular “They” (Explorations of Style)
There’s (Starting to Be) Some ‘They’ There (Lingua Franca)
Everybody Has Their Own Opinion About the Singular They (by John Lawler)
Singular “they”: everyone has their own opinion (The Economist)
They: A singular pronoun (American Heritage Dictionary)
Reflecting on the reflexive pronoun ‘themself’ (Sentence First)
Singular “their” in Jane Austen (Pemberley.com)