from an online linguistics textbook
Other reasons are often stated for prescribing language, though these may mask the ultimate political reasons. These reasons include the supposed illogical or ambiguous nature of some constructions used by people, and linguists have also sometimes gotten involved in the debate in these cases because of their expertise. An example is the use of English they, them, and their to refer to a single person, usually of unknown or unspecified gender, as in the example in the box above.
The idea is that since they is supposed to be plural, it should not be allowed to refer to one person. In cases like this, prescriptive grammarians are trying to actually improve the language or perhaps to preserve what they suppose to be an earlier, purer form of the language. The problem is that linguists and other language scientists, experts on the "logic" and the degree of ambiguity in language, can find nothing inherently wrong with any of these constructions, at least not with the usual examples of "bad" English constructions. In fact languages seem to have their own built-in "prescriptive" mechanism which weeds out whatever patterns don't work. It's a survival-of-the-fittest sort of arrangement, and it implies that those common patterns such as "singular they" (which has been around in the spoken and written language for at least 500 years) are quite fit linguistically.
The idea is that since they is supposed to be plural, it should not be allowed to refer to one person. In cases like this, prescriptive grammarians are trying to actually improve the language or perhaps to preserve what they suppose to be an earlier, purer form of the language. The problem is that linguists and other language scientists, experts on the "logic" and the degree of ambiguity in language, can find nothing inherently wrong with any of these constructions, at least not with the usual examples of "bad" English constructions. In fact languages seem to have their own built-in "prescriptive" mechanism which weeds out whatever patterns don't work. It's a survival-of-the-fittest sort of arrangement, and it implies that those common patterns such as "singular they" (which has been around in the spoken and written language for at least 500 years) are quite fit linguistically.