They is a very useful pronoun. It can be used not only as a plural, but as a singular as well. The official grammatical position on they, however, is that they can only be used after a plural subject, never singular. I have two reasons why that should be changed.

One reason is that there is simply no other effective word to take they’s place. For example, take it. It can hardly be used for a person—it indicates a lack of gender. Or try the officially accepted word—he. The person cannot simply be assumed to be male—ask and girl in my class. Using he/she addresses that issue, but quickly becomes cumbersome if used more than once. One would seem to work, but honestly, who says that? The last time I think I heard it used was when I somehow ended up watching Pride and Prejudice with my sister and her friends. None of these words work!

For example, take the title of this piece. How many other words work in place of they? “If One Doesn’t Like the Use of the Word They, One Should Read This.” That just sounds incredibly old-fashioned. “If Someone Doesn't Like the Use of the Word They, He/She Should Read This.” That sounds far too awkward. But “If Someone Doesn't Like the Use of the Word They, They Should Read This” sounds perfectly natural.
The other reason is that they is used by many people. When so many people say something, isn’t it generally accepted as correct? For example, take the book Frindle, by Andrew Clements. In the book, the main character gets so many people to call pens “frindles” that the word is eventually added to the dictionary. Or take the following footnote found in the Fourth Edition of the American Heritage College Dictionary:
“USAGE NOTE: The use of the third-person plural pronoun they to refer to a singular noun or pronoun is attested as early as 1300. The practice is found in the works of many admired writers, is widespread in mainstream publications, and is so common in speech that it generally passes unnoticed. However, despite its convenience as a substitute for phrases such as he or she, many people avoid using they to refer to a singular antecedent out of respect for the traditional grammatical rule concerning pronoun agreement...”

The only reason people avoid using they is to adhere to traditional rules, despite the convenience of words like they. When so many people use the word because it works so well, why shouldn't they be allowed to?
They is really the only word that can be used in some situations. And if so many people show they agree by using the word, why should it not be considered correct? They should be able to be used as a pronoun—both singular and plural.